<Page>
Prospectus Supplement filed pursuant to Rule 424(b)(5)
Registration Nos. 333-82230 and 333-104230
PROSPECTUS SUPPLEMENT
(To Prospectus dated February 14, 2002)
$300,000,000
[LOGO]
KEYSPAN CORPORATION
$150,000,000 4.650% Notes due 2013
$150,000,000 5.875% Notes due 2033
--------------
The notes due 2013 will bear interest at the rate of 4.650% per year and the
notes due 2033 will bear interest at the rate of 5.875% per year. Interest on
the notes is payable in arrears on April 1 and October 1 of each year, beginning
October 1, 2003. The notes due 2013 will mature on April 1, 2013 and the notes
due 2033 will mature on April 1, 2033. We may redeem some or all of the notes at
any time at the redemption price described under the caption 'Description of
Notes -- Optional Redemption.'
The notes will be senior obligations of our company and will rank equally
with all of our other unsecured senior indebtedness.
------------------
Neither the Securities and Exchange Commission nor any state commission has
approved or disapproved of these securities or determined if this prospectus is
truthful or complete. Any representation to the contrary is a criminal offense.
<Table>
<Caption>
NOTES DUE 2013 NOTES DUE 2033
----------------------- -----------------------
PER NOTE TOTAL PER NOTE TOTAL
-------- ------------ -------- ------------
Public Offering Price........................... 99.739% $149,608,500 99.763% $149,644,500
Underwriting Discount........................... 0.650% $ 975,000 0.875% $ 1,312,500
Proceeds to KeySpan (before expenses)........... 99.089% $148,633,500 98.888% $148,332,000
</Table>
The public offering prices set forth above do not include accrued interest,
if any. Interest on the notes will accrue from April 4, 2003, and must be paid
by the purchaser if the notes are delivered after April 4, 2003.
We expect that delivery of the notes will be made to purchasers through the
book-entry delivery system of The Depository Trust Company on or about April 4,
2003.
ABN AMRO INCORPORATED SALOMON SMITH BARNEY
--------------
THE ROYAL BANK OF SCOTLAND
FLEET SECURITIES, INC.
SCOTIA CAPITAL
WACHOVIA SECURITIES
April 1, 2003
<Page>
YOU SHOULD RELY ON THE INFORMATION CONTAINED IN OR INCORPORATED BY REFERENCE
IN THIS PROSPECTUS SUPPLEMENT AND THE ACCOMPANYING PROSPECTUS. WE HAVE NOT
AUTHORIZED ANYONE TO PROVIDE YOU WITH DIFFERENT INFORMATION. WE ARE NOT MAKING
AN OFFER OF THESE SECURITIES IN ANY STATE WHERE THE OFFER IS NOT PERMITTED. YOU
SHOULD NOT ASSUME THAT THE INFORMATION CONTAINED IN THIS PROSPECTUS SUPPLEMENT
OR THE ACCOMPANYING PROSPECTUS IS ACCURATE AS OF ANY DATE OTHER THAN THE DATE ON
THE FRONT OF THIS PROSPECTUS SUPPLEMENT.
-------------------
TABLE OF CONTENTS
PROSPECTUS SUPPLEMENT
<Table>
<Caption>
PAGE
----
KeySpan..................................................... S-3
Use of Proceeds............................................. S-4
Ratio of Earnings to Fixed Charges.......................... S-4
Capitalization.............................................. S-5
Description of Notes........................................ S-6
Certain United States Federal Income Tax Consequences....... S-10
Underwriting................................................ S-11
Legal Matters............................................... S-12
Experts..................................................... S-12
PROSPECTUS
PAGE
----
About this Prospectus....................................... i
Risk Factors................................................ 1
Forward-Looking Statements.................................. 3
KeySpan Corporation......................................... 4
The Trusts.................................................. 5
Use of Proceeds............................................. 5
Ratio of Earnings to Fixed Charges and of Earnings to
Combined Fixed Charges and Preferred Stock Dividends...... 5
Description of Debt Securities.............................. 7
Description of Preferred Stock.............................. 21
Description of Depositary Shares............................ 24
Description of the Trust Preferred Securities............... 27
Description of Common Stock................................. 39
Description of Stock Purchase Contracts and Stock Purchase
Units..................................................... 43
Description of Warrants and Warrant Units................... 44
United States Federal Income Tax Consequences............... 46
ERISA Considerations........................................ 63
Plan of Distribution........................................ 67
Legal Opinions.............................................. 68
Experts..................................................... 68
Where You Can Find More Information......................... 69
</Table>
S-2
<Page>
KEYSPAN
KeySpan Corporation, a New York corporation, was formed in May 1998 as a
result of the business combination of KeySpan Energy Corporation, the parent of
The Brooklyn Union Gas Company, and certain businesses of the Long Island
Lighting Company. We have assets of more than $12 billion and approximately 2.5
million gas customers throughout the Northeast.
Our core business is gas distribution, conducted by our six regulated gas
utility subsidiaries: The Brooklyn Union Gas Company d/b/a KeySpan Energy
Delivery New York and KeySpan Gas East Corporation d/b/a KeySpan Energy Delivery
Long Island distribute gas to customers in the Boroughs of Brooklyn, Queens and
Staten Island in New York City and the Counties of Nassau and Suffolk on Long
Island, respectively; Boston Gas Company, Colonial Gas Company and Essex Gas
Company, each doing business as KeySpan Energy Delivery New England, distribute
gas to customers in eastern and central Massachusetts; and EnergyNorth Natural
Gas, Inc. d/b/a KeySpan Energy Delivery New England distributes gas to customers
in central New Hampshire.
We are also a major, and growing, generator of electricity. We own and
operate five large generating plants and 46 smaller facilities in Nassau and
Suffolk Counties on Long Island and lease and operate a major facility in Queens
County in New York City. Under contractual arrangements, we provide power,
electric transmission and distribution services operations, billing and other
customer services for approximately one million electric customers of the Long
Island Power Authority on Long Island.
Our other subsidiaries are involved in oil and gas exploration and
production; gas storage; wholesale and retail gas and electric marketing;
appliance service; heating, ventilation and air conditioning installation and
services; large energy-system ownership, installation and management; and
telecommunications. We also invest in, and participate in the development of,
pipelines and other energy-related projects, domestically and internationally.
We are a registered holding company under the Public Utility Holding Company
Act of 1935, as amended, or PUHCA. Therefore, our corporate and financial
activities and those of our subsidiaries, including their ability to pay
dividends to us, are subject to regulation by the SEC. Under our holding company
structure, we have no independent operations or sources of income of our own and
conduct substantially all of our operations through our subsidiaries and, as a
result, we depend on the earnings and cash flow of, and dividends or
distributions from, our subsidiaries to provide the funds necessary to meet our
debt and contractual obligations and to pay dividends on our common stock.
Furthermore, a substantial portion of our consolidated assets, earnings and cash
flow is derived from the operations of our regulated utility subsidiaries, whose
legal authority to pay dividends or make other distributions to us is subject to
regulation by state regulatory authorities.
Our principal place of business is One MetroTech Center, Brooklyn, New York
11201, and our telephone number is (718) 403-1000.
S-3
<Page>
USE OF PROCEEDS
The net proceeds from the offering will be used to repay approximately
$297 million of commercial paper that was issued to redeem long-term debt. The
commercial paper currently bears interest at a weighted average annualized
interest rate of 1.37% and has maturities ranging from 2 to 84 days.
RATIO OF EARNINGS TO FIXED CHARGES
The ratio of earnings to fixed charges for each of the periods indicated is
as follows:
<Table>
<Caption>
YEARS ENDED DECEMBER 31,
NINE MONTHS ENDED ---------------------------------------------
DECEMBER 31, 1998 1999 2000 2001 2002
- ----------------- ---- ---- ---- ----
(1) 3.23 3.04 2.10 2.66
</Table>
- ---------
<Table>
(1) For the nine months ended December 31, 1998, earnings were
insufficient to cover fixed charges by $365.0 million.
During the nine months ended December 31, 1998, KeySpan
incurred the following special charges (after tax): charges
associated with the sale to Long Island Power Authority of
the electric transmission and distribution system of $107.9
million; charges associated with the combination of Long
Island Lighting Company's gas and electric services
businesses with KeySpan of $83.5 million; an impairment
charge of $54.1 million to write down the value of proved
gas reserves; and a charge of $13.0 million to establish a
not- for-profit philanthropic foundation.
</Table>
S-4
<Page>
CAPITALIZATION
The following table sets forth as of December 31, 2002:
our actual short-term debt and capitalization; and
our short-term debt and capitalization as adjusted to
reflect (1) the issuance of 13,900,000 shares of common
stock in January 2003 and the use of the net proceeds
thereof of $473 million to pay down short-term debt and (2)
the redemption of $443 million of promissory notes (net of
unamortized discount of $4 million) in March 2003 with the
proceeds from the issuance of commercial paper, including
the net income effect of $5 million related to the payment
of the related call premiums, a gain on a related interest
rate swap, and the unamortized discount; and
our short-term debt and capitalization as further adjusted
to reflect the issuance of the notes and the use of the
proceeds thereof to pay down short-term debt as described
under the caption 'Use of Proceeds.'
You should read this table in conjunction with our consolidated financial
statements and the notes to these consolidated financial statements incorporated
by reference in this prospectus supplement.
<Table>
<Caption>
AT DECEMBER 31, 2002
----------------------------------------------
AS ADJUSTED FOR
ISSUANCE OF
AS ADJUSTED FOR COMMON STOCK,
ISSUANCE OF REDEMPTION OF
COMMON STOCK PROMISSORY
AND REDEMPTION NOTES AND
OF PROMISSORY ISSUANCE OF
ACTUAL NOTES NOTES
---------- --------------- ---------------
(IN THOUSANDS)
Short-term debt, including current maturities of
long-term debt.................................... $ 927,110 $ 901,142 $ 604,276
---------- ---------- ----------
---------- ---------- ----------
Long-term debt (1).................................. 5,224,081 4,780,930 4,780,930
Notes due 2013...................................... 149,609
Notes due 2033...................................... 149,644
---------- ---------- ----------
Total long-term debt............................ 5,224,081 4,780,930 5,080,183
---------- ---------- ----------
Preferred Stock..................................... 83,849 83,849 83,849
---------- ---------- ----------
Common stock, $.01 par value; authorized 450,000,000
shares; outstanding 142,424,774 shares, actual,
and 156,324,774 shares, as adjusted for issuance
of common stock (2)............................... 3,005,354 3,478,327 3,478,327
Retained earnings................................... 522,835 517,704 517,704
Accumulated comprehensive income.................... (108,423) (108,423) (108,423)
Treasury stock purchased............................ (475,174) (475,174) (475,174)
---------- ---------- ----------
Total common shareholders' equity............... 2,944,592 3,412,434 3,412,434
---------- ---------- ----------
Total short-term debt and capitalization (3).... $9,179,632 $9,178,355 $9,180,742
---------- ---------- ----------
---------- ---------- ----------
</Table>
- ---------
(1) Long-term debt includes $460 million of MEDS Equity Units.
(2) The number of outstanding shares does not include the common
stock issuable upon exercise of outstanding options,
warrants and other convertible securities or upon settlement
of the purchase contracts underlying our MEDS Equity Units.
(3) Consists of Short-term debt, Long-term debt, Preferred stock
and Total common stockholders' equity.
S-5
<Page>
DESCRIPTION OF NOTES
This description of the terms of the notes supplements the description of
the general terms and provisions of the securities in the accompanying
prospectus. If this summary differs in any way from that description in the
accompanying prospectus, you should rely on this summary. The notes will be
issued as separate series of debt securities, which we have registered for
issuance on terms to be determined at the time of their sale. Whenever we refer
in this 'Description of Notes' to terms defined in the indenture, we intend that
the defined terms be incorporated in this description by reference.
GENERAL
The notes will be issued under an indenture, dated as of November 1, 2000,
between us and JPMorgan Chase Bank, as trustee, as supplemented.
The notes due 2013 will initially be issued in an aggregate principal amount
of $150,000,000 and the notes due 2033 will initially be issued in an aggregate
principal amount of $150,000,000. We may, without consent of the holders of the
notes, create and issue additional notes ranking equally with the notes and
otherwise similar in all respects except for the issue date and issue price so
that such additional notes shall be consolidated and form a single series with
the notes due 2013 or the notes due 2033, as the case may be. No additional
notes can be issued if an Event of Default has occurred with respect to the
notes.
The notes will be issued in minimum denominations of $1,000 and in integral
multiples of $1,000.
The notes will be our unsecured obligations and will rank senior to our
future debt that is subordinated to the notes, and will rank equally with our
debt that is not subordinated to the notes.
Any money we pay to a paying agent for the payment of principal of, premium,
or interest on the notes which remains unclaimed for two years after the date
the payment was due will be returned to us. Upon the return of those moneys to
us, holders of the notes, with respect to moneys so returned, must look to us
for payment as our unsecured general creditors and any liability of the paying
agent with respect to those moneys will cease.
We are a holding company and derive all of our income from subsidiaries. Our
rights and the rights of our creditors, including holders of the notes, to
participate in the distribution of assets of any of our subsidiaries upon its
liquidation, reorganization or otherwise will be subject to the prior claims of
the creditors and preferred shareholders of that subsidiary. As of December 31,
2002, the aggregate debt of our subsidiaries was approximately $2 billion.
MATURITY AND INTEREST
The notes due 2013 will mature on April 1, 2013 and will bear interest at
the annual rate of 4.650%. The notes due 2033 will mature on April 1, 2033 and
will bear interest at the annual rate of 5.875%. Interest will be payable in
arrears on April 1 and October 1 of each year, beginning October 1, 2003.
Interest on the notes will be paid to holders of record on the March 15 and
September 15 preceding the related interest payment date.
Interest payments in respect of the notes will equal the amount of interest
accrued from and including the immediately preceding interest payment date in
respect of which interest has been paid or duly made available for payment (or
from and including the date of issue, if no interest has been paid or duly made
available for payment with respect to the notes) to but excluding the applicable
interest or principal payment date, as the case may be. Interest on the notes
will be computed on the basis of a 360-day year of twelve 30-day months.
If any interest or principal payment date falls on a day that is not a
business day, the required payment of principal and/or interest will be made on
the next business day as if made on the date that payment was due, and no
interest will accrue on that payment for the period from and after the interest
or principal payment date, as the case may be, to the date of the payment
S-6
<Page>
on the next business day. As used in this prospectus supplement, 'business day'
means any day, other than a Saturday or Sunday, that is neither a legal holiday
nor a day on which commercial banks are authorized or required by law,
regulation or executive order to close in The City of New York.
OPTIONAL REDEMPTION
The notes may be redeemed in whole at any time or in part from time to time,
at our option, on not less than 30 nor more than 60 days' prior notice, prior to
their maturity at a redemption price equal to the sum of the principal amount of
the notes, the Make-Whole Amount described below and any accrued and unpaid
interest to the date of redemption. Holders of record on a record date that is
on or prior to a redemption date will be entitled to receive interest due on the
interest payment date.
The term 'Make-Whole Amount' means, the excess, if any, of:
the aggregate present value as of the date of the redemption
of principal being redeemed and the amount of interest
(exclusive of interest accrued to the date of redemption)
that would have been payable if redemption had not been
made, determined by discounting, on a semiannual basis, the
remaining principal and interest at the Reinvestment Rate
described below (determined on the third business day
preceding the date notice of redemption is given) from the
dates on which the principal and interest would have been
payable if the redemption had not been made, to the date of
redemption, over
the aggregate principal amount of the notes being redeemed.
The term 'Reinvestment Rate' means 15 basis points, in the case of the notes
due 2013, and 20 basis points, in the case of the notes due 2033, in each case
plus the arithmetic mean of the yield under the heading 'Week Ending' published
in the most recent Statistical Release under the caption 'Treasury Constant
Maturities' for the maturity (rounded to the nearest month) corresponding to the
remaining life to maturity, as of the payment date, of the notes being redeemed
or paid. If no maturity exactly corresponds to the remaining life to maturity of
the notes being redeemed, yields for the two published maturities most closely
corresponding to the maturity would be so calculated and the Reinvestment Rate
would be interpolated or extrapolated on a straight-line basis, rounding to the
nearest month.
The term 'Statistical Release' means the statistical release designated
'H.15(519)' or any successor publication which is published weekly by the
Federal Reserve System and which establishes yields on actively-traded United
States government securities adjusted to constant maturities, or, if such
statistical release is not published at the time of any determination under the
terms of the notes, then such other reasonably comparable index which will be
designated by us.
The most recent Statistical Release published prior to the date of
determination of the Make-Whole Amount will be used for purposes of calculating
the Reinvestment Rate.
The Make-Whole Amount will be calculated by an independent investment
banking institution of national standing appointed by us. If the Reinvestment
Rate is not available as described above, the Reinvestment Rate will be
calculated by interpolation or extrapolation of comparable rates selected by the
independent investment banking institution.
BOOK ENTRY, DELIVERY AND FORM
The notes will be represented by one or more global securities that will be
deposited with and registered in the name of The Depository Trust Company
('DTC') or its nominee. Thus, we will not issue certificated securities to you
for the notes, except in the limited circumstances described below. Each global
security will be issued to DTC, which will keep a computerized record of its
participants whose clients have purchased the notes. Each participant will then
keep a record of its clients. Unless it is exchanged in whole or in part for a
certificated security, a global security may not be transferred. DTC, its
nominees and their successors may, however, transfer a global security
S-7
<Page>
as a whole to one another, and these transfers are required to be recorded or a
register to be maintained by the trustee.
Beneficial interests in a global security will be shown on, and transfers of
beneficial interests in the global security will be made only through, records
maintained by DTC and its participants. DTC has provided us with the following
information: DTC is a limited-purpose trust company organized under the New York
Banking Law, a 'banking organization' within the meaning of the New York Banking
Law, a member of the United States Federal Reserve System, a 'clearing agency'
registered under the provisions of Section 17A of the Securities Exchange Act of
1934. DTC holds securities that its direct participants deposit with DTC. DTC
also records the settlements among direct participants of securities
transactions, such as transfers and pledges, in deposited securities through
computerized records for direct participants' accounts. This eliminates the need
to exchange certificated securities. Direct participants include securities
brokers and dealers, banks, trusts companies, clearing corporations and certain
other organizations.
DTC's book-entry system is also used by other organizations such as
securities brokers and dealers, banks and trust companies that work through a
direct participant. The rules that apply to DTC and its participants are on file
with the SEC.
DTC is owned by a number of its direct participants and by the New York
Stock Exchange, Inc., the American Stock Exchange, Inc. and the National
Association of Securities Dealers, Inc.
When you purchase notes through the DTC system, the purchases must be made
by or through a direct participant, which will receive credit for the notes on
DTC's records. When you actually purchase the notes, you will become their
beneficial owner. Your ownership interest will be recorded only on the direct or
indirect participants' records. DTC will have no knowledge of your individual
ownership of the notes. DTC's records will show only the identity of the direct
participants and the amount of notes held by or through them. You will not
receive a written confirmation of your purchase or sale or any periodic account
statement directly from DTC. You should instead receive these from your direct
or indirect participant. As a result, the direct or indirect participants are
responsible for keeping accurate account of the holdings of their customers. The
trustee will wire payments on the notes to DTC's nominee. The trustee and we
will treat DTC's nominee as the owner of each global security for all purposes.
Accordingly, the trustee, any paying agent and we will have no direct
responsibility or liability to pay amounts due on a global security to you or
any other beneficial owners in that global security. Any redemption notices will
be sent by us directly to DTC, which will, in turn, inform the direct
participants (or the indirect participants), which will then contact you as a
beneficial holder.
It is DTC's current practice, upon receipt of any payment of distributions
or liquidation amounts, to proportionately credit direct participants' accounts
on the payment date based on their holdings. In addition, it is DTC's current
practice to pass through any consenting or voting rights to such participants by
using an omnibus proxy. Those participants will, in turn, make payments to and
solicit votes from you, the ultimate owner of notes, based on the customary
practices. Payments to you will be the responsibility of the participants and
not of DTC, the trustee or our company.
Notes represented by one or more global securities will be exchangeable for
certificated securities with the same terms in authorized denominations only if:
DTC is unwilling or unable to continue as depositary or
ceases to be a clearing agency registered under applicable
law, and a successor is not appointed by us within 90 days;
or
we decide to discontinue the book-entry system; or
an event of default has occurred and is continuing with
respect to the notes.
If the global security is exchanged for certificated securities, the trustee
will keep the registration books for the notes at its corporate office and
follow customary practice and procedures regarding those certificated
securities.
S-8
<Page>
CLEARSTREAM BANKING AND EUROCLEAR
Links have been established among DTC and Clearstream Banking societe
anonyme and Euroclear Bank S.A./N.V., as operator of the Euroclear System, which
are two European clearing systems, to facilitate the initial issuance of the
notes sold outside of the United States and cross-market transfers of the notes
associated with secondary market trading.
Although DTC, Clearstream Banking and Euroclear have agreed to the
procedures provided below in order to facilitate transfers, they are under no
obligation to perform these procedures, and these procedures may be modified or
discontinued at any time.
Clearstream Banking and Euroclear will record the ownership interests of
their participants in much the same way as DTC, and DTC will record the total
ownership of each of the U.S. agents of Clearstream Banking and Euroclear, as
participants in DTC.
When notes are to be transferred from the account of a DTC participant to
the account of a Clearstream Banking participant or a Euroclear participant, the
purchaser must send instructions to Clearstream Banking or Euroclear through a
participant at least one day prior to settlement. Clearstream Banking or
Euroclear, as the case may be, will instruct its U.S. agent to receive notes
against payment. After settlement, Clearstream Banking or Euroclear will credit
its participant's account. Credit for the notes will appear on the next day
(European time).
Because settlement is taking place during New York business hours, DTC
participants will be able to employ their usual procedures for sending notes to
the relevant U.S. agent acting for the benefit of Clearstream Banking or
Euroclear participants. The sale proceeds will be available to the DTC seller on
the settlement date. As a result, to the DTC participant, a cross-market
transaction will settle no differently than a trade between two DTC
participants.
When a Clearstream Banking or Euroclear participant wishes to transfer notes
to a DTC participant, the seller will be required to send instructions to
Clearstream Banking or Euroclear through a participant at least one business day
prior to settlement. In these cases, Clearstream Banking or Euroclear will
instruct its U.S. agent to transfer these notes against payment for them. The
payment will then be reflected in the account of the Clearstream Banking or
Euroclear participant the following day, with the proceeds back-valued to the
value date, which would be the preceding day, when settlement occurs in New
York. If settlement is not completed on the intended value date, that is, the
trade fails, proceeds credited to the Clearstream Banking or Euroclear
participant's account will instead be valued as of the actual settlement date.
S-9
<Page>
CERTAIN UNITED STATES FEDERAL INCOME TAX CONSEQUENCES
A summary of certain United States federal income tax consequences that will
apply to holders of debt securities is set forth under 'United States Federal
Income Tax Consequences -- Debt securities' in the Prospectus. The sections
below replace the summaries set forth in the Prospectus under 'United States
Federal Income Tax Consequences -- Debt securities -- Consequences to non-United
States holders -- United States federal estate tax' and 'United States Federal
Income Tax Consequences -- Debt securities -- Information reporting and backup
withholding -- Non-United States holders.'
CONSEQUENCES TO NON-UNITED STATES HOLDERS
United States federal estate tax
Your estate will not be subject to United States federal estate tax on debt
securities beneficially owned by you at the time of your death provided that:
any payment to you on the debt securities would be eligible
for exemption from the 30% United States federal withholding
tax under the rules described in the bullet points under
'United States Federal Income Tax Consequences -- Debt
securities -- Consequences to non-United States
holders -- United States federal withholding tax,' without
regard to the certification requirements of the fourth
bullet point; and
interest on those debt securities would not have been, if
received at the time of your death, effectively connected
with the conduct by you of a trade or business in the United
States.
INFORMATION REPORTING AND BACKUP WITHHOLDING
Non-United States holders
If you are a non-United States holder of debt securities, we must report
annually to the IRS and to you the amount of payments we make to you and the tax
withheld with respect to such payments, regardless of whether withholding was
required. Copies of the information returns reporting such payments and
withholding may also be made available to the tax authorities in the country in
which you reside under the provisions of an applicable income tax treaty. You
will not be subject to backup withholding regarding payments we make to you
provided that we do not have actual knowledge or reason to know that you are a
United States person and we have received from you the statement described above
in the fourth bullet point under 'United States Federal Income Tax
Consequences -- Debt securities -- Consequences to non-United States holders --
United States federal withholding tax.'
In addition, you will be subject to information reporting and, depending on
the circumstances, backup withholding regarding the proceeds of the sale of a
debt security made within the United States or conducted through United
States-related intermediaries, unless the payor receives the statement described
above and does not have actual knowledge or reason to know that you are a United
States person, or you otherwise establish an exemption.
Any amounts withheld under the backup withholding rules will be allowed as a
refund or a credit against your United States federal income tax liability
provided the required information is furnished to the IRS.
S-10
<Page>
UNDERWRITING
ABN AMRO Incorporated and Salomon Smith Barney Inc. are acting as joint
book-running managers of the offering, and representatives of the underwriters
named below.
Subject to the terms and conditions stated in the underwriting agreement
dated the date of this prospectus supplement, each underwriter named below has
agreed to purchase, and we have agreed to sell to that underwriter, the
respective principal amount of notes set forth opposite the underwriter's name.
<Table>
<Caption>
PRINCIPAL AMOUNT PRINCIPAL AMOUNT
UNDERWRITER OF NOTES DUE 2013 OF NOTES DUE 2033
----------- ----------------- -----------------
ABN AMRO Incorporated....................................... $ 45,000,000 $ 45,000,000
Salomon Smith Barney Inc. .................................. 45,000,000 45,000,000
The Royal Bank of Scotland plc.............................. 30,000,000 30,000,000
Fleet Securities, Inc. ..................................... 10,000,000 10,000,000
Scotia Capital (USA) Inc. .................................. 10,000,000 10,000,000
Wachovia Securities, Inc. .................................. 10,000,000 10,000,000
------------ ------------
Total................................................... $150,000,000 $150,000,000
------------ ------------
------------ ------------
</Table>
The underwriting agreement provides that the obligations of the underwriters
to purchase the notes included in this offering are subject to approval of legal
matters by counsel and to other conditions. The underwriters are obligated to
purchase all the notes if they purchase any of the notes.
The notes are a new issue of securities with no established trading market
and will not be listed on any national securities exchange. The underwriters
have advised us that they intend to make a market for the notes, but they have
no obligation to do so and may discontinue market making at any time without
providing any notice. No assurance can be given as to the liquidity of any
trading market for the notes.
The underwriters propose to offer some of the notes directly to the public
at the respective public offering prices set forth on the cover page of this
prospectus and some of the notes to dealers at the public offering price less a
concession not to exceed 0.4% of the notes due 2013 and 0.5% of the notes due
2033. The underwriters may allow, and dealers may reallow a concession not to
exceed 0.25% of the principal amount of the notes on sale to other dealers.
After the initial offering of the notes to the public, the representatives may
change the public offering price and concessions.
The following table shows the underwriting discounts and commissions that we
are to pay to the underwriters in connection with this offering (expressed as a
percentage of the principal amount of the notes).
<Table>
<Caption>
PAID BY US
----------
Per note due 2013........................................... 0.650%
Per note due 2033........................................... 0.875%
</Table>
In connection with the offering, the underwriters may purchase and sell
notes in the open market. These transactions may include over-allotment,
syndicate covering transactions and stabilizing transactions. Over-allotment
involves syndicate sales of notes in excess of the principal amount of notes to
be purchased by the underwriters in the offering, which creates a syndicate
short position. Syndicate covering transactions involve purchases of the notes
in the open market after the distribution has been completed in order to cover
syndicate short positions. Stabilizing transactions consist of certain bids or
purchases of notes made for the purpose of preventing or retarding a decline in
the market price of the notes while offering is in progress.
The underwriters also may impose a penalty bid. Penalty bids permit the
underwriters to reclaim a selling concession from a syndicate member when the
underwriters, in covering syndicate
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short positions or making stabilizing purchases, repurchase notes originally
sold by that syndicate member.
Any of these activities may have the effect of preventing or retarding a
decline in the market price of the notes. They may also cause the price of the
notes to be higher than the price that otherwise would exist in the open market
in the absence of these transactions. The underwriters may conduct these
transactions in the over-the-counter market or otherwise. If the underwriters
commence any of these transactions, they may discontinue them at any time.
We estimate that our total expense associated with this offering, to be paid
by us, will be $100,000.
The underwriters and/or affiliates of the underwriters have performed
commercial and investment banking and advisory services for us from time to time
for which they have received customary fees and expenses. The underwriters
and/or affiliates of the underwriters may in the future engage in transactions
with and perform services for us in the ordinary course of their business, for
which they would receive customary fees and expenses.
We have agreed to indemnify the underwriters against certain liabilities,
including liabilities under the Securities Act of 1933, as amended, or to
contribute to payments the underwriters may be required to make because of any
of those liabilities.
LEGAL MATTERS
The validity of the notes will be passed upon for us by Simpson Thacher &
Bartlett, New York, New York, and for the underwriters by Davis Polk & Wardwell,
New York, New York.
EXPERTS
The consolidated financial statements and the related financial statement
schedule as of and for the year ended December 31, 2002 incorporated in this
prospectus supplement from our annual report on Form 10-K for the year ended
December 31, 2002 have been audited by Deloitte & Touche LLP, independent
auditors, as stated in their report, which is incorporated herein by reference
(which report expresses an unqualified opinion and includes explanatory
paragraphs related to the adoption of Statement of Financial Accounting
Standards No. 142, 'Goodwill and Other Intangible Assets' and related revisions
made to Note 1G and revisions made to Note 12, related to subsidiary guarantor
information, related to the 2001 and 2000 consolidated financial statements that
were audited by other auditors who have ceased operations and for which Deloitte
& Touche has expressed no opinion or other form of assurance other than with
respect to such disclosures), and has been so incorporated in reliance upon the
report of such firm given upon their authority as experts in accounting and
auditing.
The consolidated financial statements as of December 31, 2001 and for the
years ended December 31, 2001 and December 31, 2000 incorporated in this
prospectus supplement from our annual report on Form 10-K for the year ended
December 31, 2002 have been audited by Arthur Andersen LLP, independent public
accountants, as stated in their report, which is incorporated herein by
reference, and has been so incorporated in reliance upon the report of such firm
given upon their authority as experts in accounting and auditing. Arthur
Andersen has not consented to the inclusion of their report in the most recent
registration statement on Form S-3 filed with SEC covering the notes. Holders of
notes may have no effective remedy against Arthur Andersen in connection with a
material misstatement or omission in those financial statements.
Information related to the estimated proved reserves attributable to certain
oil and gas properties of our subsidiaries as of December 31, 2002 and estimates
of future net cash flows and present value of the reserves have been
incorporated by reference in our Annual Report on Form 10-K for the year ended
December 31, 2002, which is incorporated herein by reference, in reliance on the
reserve report, dated January 20, 2003, prepared by Netherland, Sewell &
Associates, Inc., independent petroleum consultants, and the reserve report,
dated February 5, 2003 prepared by Miller and Lents, Ltd., independent oil and
gas consultants.
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________________________________________________________________________________
$300,000,000
[LOGO]
KEYSPAN CORPORATION
$150,000,000 4.650% Notes due 2013
$150,000,000 5.875% Notes due 2033
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PROSPECTUS SUPPLEMENT
April 1, 2003
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ABN AMRO INCORPORATED
SALOMON SMITH BARNEY
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THE ROYAL BANK OF SCOTLAND
FLEET SECURITIES, INC.
SCOTIA CAPITAL
WACHOVIA SECURITIES
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