EXHIBIT 8.1

October 4, 2001

                   [CADWALADER, WICKERSHAM & TAFT LETTERHEAD]


DIANON Systems, Inc.
200 Watson Boulevard
Stratford, CT 06615

Re:      Merger of UroCor, Inc. with and into DIANON Systems, Inc.
         ---------------------------------------------------------

Ladies and Gentlemen:

          You have asked us for our opinion regarding certain U.S. federal
income tax matters in connection with the merger (the "Merger") of UroCor, Inc.
(the "Company"), a Delaware corporation, with and into DIANON Systems, Inc.
("Acquirer"), a Delaware corporation, to be accomplished pursuant to the
Agreement and Plan of Merger dated June 28, 2001 (the "Original Merger
Agreement"), by and among Acquirer, UroCor Acquisition Corp., a Delaware
corporation and a direct wholly-owned subsidiary of Acquirer ("Merger Sub") and
the Company, as amended by that certain Amendment No. 1 to Agreement and Plan of
Merger dated as of October 3, 2001 (the "First Amendment"), by and among
Acquirer, Merger Sub and the Company (the Original Merger Agreement as amended
by the First Amendment is referred to herein as the "Merger Agreement"). All
capitalized terms used but not defined herein shall have the meaning ascribed to
such terms in the Merger Agreement.

          In rendering the opinions expressed below, we have examined and relied
upon (i) the Merger Agreement, (ii) the Registration Statement on Form S-4, as
filed by Acquirer with the Securities and Exchange Commission (the "Commission")
under the Securities Act of 1933, as amended (the "Act"), including the related
Joint Proxy Statement/Prospectus of Acquirer and the Company (the "Registration
Statement"), (iii) documents delivered in connection with the transactions
contemplated by the Merger Agreement including, without limitation, the
representation letters made by an authorized officer of each of Acquirer and the
Company, dated as of the date hereof and addressed to us, and (iv) all such
documents, instruments and other certificates as we have deemed appropriate as a
basis for the opinions expressed below (collectively with all documents referred
to in this sentence, the "Transaction Documents").




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          In rendering the opinions set forth below, we have relied upon all
statements, facts and representations in the Transaction Documents, and assumed
that all such documents are complete and authentic and have been duly
authorized, executed and delivered. We have further assumed that all statements,
facts and representations made in such documents are true (without regard to any
qualifications stated therein and without undertaking to verify such statements,
facts and representations by independent investigation), that the respective
parties thereto and all parties referred to therein will act in all respects and
at all relevant times in conformity with the requirements and provisions of such
documents, and that none of the terms and conditions contained therein has been
or will be waived or modified in any respect. We have made such other
investigations of fact and law as we have deemed appropriate as a basis for the
opinions expressed below.

          The following opinions are based upon the Internal Revenue Code of
1986, as amended (the "Code"), applicable Treasury regulations, and rulings and
decisions thereunder, each as in effect on the date hereof, and may be affected
by amendments to the Code or to Treasury regulations thereunder or by subsequent
judicial or administrative interpretation thereof, any of which may have
retroactive effect. We express no opinions other than as to the federal income
tax law of the United States of America. This opinion letter does not address
the various state, local or foreign tax consequences that may result from the
transactions contemplated by the Merger Agreement.

          On the basis of and subject to the foregoing, it is our opinion, as
of the date hereof and under existing law, that

1.        the consummation of the Merger will be treated for U.S. federal
          income tax purposes as a reorganization within the meaning of Section
          368(a) of the Code,

2.        Acquirer and the Company will each be a party to the reorganization
          within the meaning of Section 368(b) of the Code.

          We hereby consent to the filing of this opinion letter with the
Commission as an exhibit to the Registration Statement. We also consent to the
reference to our firm under the headings "The Merger - Material Federal Income
Tax Consequences of the Merger" and "Certain Legal Information - Legal Matters"
in the Joint Proxy Statement/Prospectus. In giving this consent, we do not admit
that we are in the category of persons whose consent is required under Section 7
of the Act or the General Rules and Regulations of the Commission.

          This opinion letter is furnished to you solely for your benefit and is
not to be used, circulated, quoted or otherwise referred to for any other
purpose or relied upon by any other person without our express written
permission.


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         We expressly disclaim any obligation or undertaking to update or modify
this opinion letter as a consequence of any future changes in applicable laws or
Treasury regulations or the facts bearing upon this opinion letter, any of which
could affect our conclusions.

Very truly yours,


/s/ Cadwalader, Wickersham & Taft

CADWALADER, WICKERSHAM & TAFT