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Exhibit 99.1

UNAUDITED, PRO FORMA COMBINED FINANCIAL INFORMATION

        The following unaudited, pro forma combined financial information describes the pro forma effect of our proposed transaction with ILEX Oncology, Inc., or ILEX, on our statements of operations for the year ended December 31, 2003 and the six months ended June 30, 2004, as if the transaction had occurred on January 1, 2003, and our balance sheet as of June 30, 2004, as if the transaction had occurred on this date. Certain historical financial information of ILEX has been reclassified to conform with the presentation of our historical financial statements.

        This pro forma combined financial information is for informational purposes only. It does not purport to indicate the results that would have actually been obtained had the transaction been completed on the assumed date or for the periods presented, or which may be obtained in the future. To produce the pro forma financial information, we allocated the purchase price using our best estimates. The unaudited, pro forma combined financial statements should be read in conjunction with the historical consolidated financial statements, including notes thereto, of both Genzyme and ILEX. Our historical consolidated financial statements are included in Exhibit 13.1 to our annual report on Form 10-K for the year ended December 31, 2003, filed with the Securities and Exchange Commission, or SEC, on March 15, 2004 and our quarterly report on Form 10-Q for the quarterly period ended June 30, 2004, filed with the SEC on August 9, 2004. The historical consolidated financial statements of ILEX are included in pages F-1 through F-31 of ILEX's annual report on Form 10-K for the year ended December 31, 2003, filed with the SEC on March 12, 2004 and pages 3 through 10 of ILEX's quarterly report on Form 10-Q for the quarterly period ended June 30, 2004, filed with the SEC on July 30, 2004.

        The merger agreement between Genzyme and ILEX provides that one of our subsidiaries would be merged with and into ILEX, which would then be merged into another one of our wholly-owned subsidiaries. In the transaction, each outstanding share of ILEX common stock would be converted into the right to receive a fraction of a share of our common stock, which we refer to as Genzyme Stock. We refer to this fraction as the "exchange ratio," and it would be calculated by dividing $26.00 by the average per share closing price of Genzyme Stock as reported by the NASDAQ National Market over the 20 trading days ending on the fifth trading day prior to the closing of the transaction, except that if the average closing price is greater than $59.88, the exchange ratio would be 0.4342, and if the average closing price is less than $46.58, the exchange ratio would be 0.5582. Cash would be paid for any fractional shares that result from conversion of ILEX common stock to Genzyme Stock. Additionally, each option to purchase shares of ILEX common stock outstanding immediately before the effective date of the merger would be assumed by us after the merger and would be exchanged for a fully vested option to purchase shares of Genzyme Stock. The transaction would be accounted for as a purchase and is expected to qualify as a tax-free reorganization.

        We have prepared the unaudited, pro forma combined financial information using the purchase method of accounting for the transaction. We expect to have reorganization and restructuring expenses as well as potential operating efficiencies as a result of the transaction. The unaudited, pro forma combined financial statements and related notes do not reflect these potential expenses and efficiencies.

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GENZYME CORPORATION
UNAUDITED, PRO FORMA COMBINED STATEMENTS OF OPERATIONS
For the Year Ended December 31, 2003
(Amounts in thousands)

				Historical Genzyme Corporation			Historical ILEX Oncology, Inc.			Pro Forma Adjustments			Footnote Reference	Pro Forma Genzyme Corporation
Revenues:
	Net product sales			$	1,563,509		$	—		$	—			$	1,563,509
	Product profit and royalty				—			23,315			—				23,315
	Net service sales				130,984			—			—				130,984
	Outlicensing revenue				—			2,773			—				2,773
	Revenues from research and development contracts:
		Related parties			1,836			—			—				1,836
		Other			17,542			8,701			—				26,243
			Total revenues		1,713,871			34,789			—				1,748,660
Operating costs and expenses:
	Cost of products sold				399,961			—			—				399,961
	Cost of services sold				75,683			—			—				75,683
	Selling, general and administrative				519,977			14,713			—				534,690
	Research and development (including research and development related to contracts)				335,256			60,693			—				395,949
	Amortization of intangibles				80,257			4,945			19,113		[A1]		104,315
	Purchase of in-process research and development				158,000			—			—				158,000
	Charge for impaired goodwill				102,792			—			—				102,792
	Charge for impaired assets				10,894			213			—				11,107
	Settlement charge				—			16,500			—				16,500
			Total operating costs and expenses		1,682,820			97,064			19,113				1,798,997
Operating income (loss)					31,051			(62,275	)		(19,113	)			(50,337	)
Other income (expenses):
	Equity in loss of equity method investments				(16,743	)		—			—				(16,743	)
	Minority interest				2,232			—			—				2,232
	Loss on investments in equity securities				(1,201	)		—			—				(1,201	)
	Loss on sale of product line				(27,658	)		—			—				(27,658	)
	Other				959			18			—				977
	Investment income				43,015			4,304			—				47,319
	Interest expense				(26,600	)		(4,137	)		—				(30,737	)
			Total other income (expenses)		(25,996	)		185			—				(25,811	)
Income (loss) before income taxes					5,055			(62,090	)		(19,113	)			(76,148	)
(Provision for) benefit from income taxes					(72,647	)		(24	)		29,907		[A2]		(42,764	)
Net income (loss)				$	(67,592	)	$	(62,114	)	$	10,794			$	(118,912	)

See Notes to Unaudited, Pro Forma Combined Financial Statements.

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GENZYME CORPORATION
UNAUDITED, PRO FORMA COMBINED STATEMENTS OF OPERATIONS
For the Year Ended December 31, 2003
(Amounts in thousands, except per share data)

			Historical Genzyme Corporation			Historical ILEX Oncology, Inc.			Pro Forma Adjustments		Footnote Reference	Pro Forma Genzyme Corporation
Net income (loss) per share:
Allocated to Genzyme Stock(1,2):
	Genzyme General net income (loss)		$	82,143		$	(62,114	)	$	10,794	[A3]	$	30,823
	Tax benefit allocated from Genzyme Biosurgery			8,720			—			—			8,720
	Tax benefit allocated from Genzyme Molecular Oncology			3,420			—			—			3,420
	Net income (loss) allocated to Genzyme Stock		$	94,283		$	(62,114	)	$	10,794		$	42,963
	Net income per share of Genzyme Stock:
		Basic	$	0.43								$	0.18
		Diluted	$	0.42								$	0.18
	Weighted average shares outstanding:
		Basic		219,376									237,755
		Diluted		225,419									244,083
Allocated to Biosurgery Stock (through June 30, 2003)(1,2):
	Genzyme Biosurgery net loss		$	(166,656	)							$	(166,656	)
	Allocated tax benefit			14,005									14,005
	Net loss allocated to Biosurgery Stock		$	(152,651	)							$	(152,651	)
	Net loss per share of Biosurgery Stock—basic and diluted		$	(3.76	)							$	(3.76	)
	Weighted average shares outstanding			40,630									40,630
Allocated to Molecular Oncology Stock (through June 30, 2003)(1,2):
	Net loss allocated to Molecular Oncology Stock		$	(9,224	)							$	(9,224	)
	Net loss per share of Molecular Oncology Stock—basic and diluted		$	(0.54	)							$	(0.54	)
	Weighted average shares outstanding			16,958									16,958

(1)

Effective July 1, 2003, in connection with the elimination of its tracking stock structure, we ceased allocating earnings to Genzyme Biosurgery and Genzyme Molecular Oncology. From that date forward, all of our earnings are allocated to Genzyme General. Earnings or losses allocated to Genzyme Biosurgery and Genzyme Molecular Oncology prior to July 1, 2003 remain allocated to those divisions and are not affected by the elimination of our tracking stock structure.

(2)

From July 1, 2003 through May 27, 2004, we referred to our outstanding series of common stock as Genzyme General Stock. At our annual meeting of shareholders on May 27, 2004, our shareholders approved an amendment to our charter that eliminated the designation of separate series of common stock, resulting in 690,000,000 authorized shares of a single series of common stock, which we refer to as Genzyme Stock.

See Notes to Unaudited, Pro Forma Combined Financial Statements.

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GENZYME CORPORATION
UNAUDITED, PRO FORMA COMBINED STATEMENTS OF OPERATIONS
For the Six Months Ended June 30, 2004
(Amounts in thousands, except per share amounts)

			Historical Genzyme Corporation			Historical ILEX Oncology, Inc.			Pro Forma Adjustments			Footnote Reference	Pro Forma Genzyme Corporation
Revenues:
	Net product sales		$	945,629		$	—		$	—			$	945,629
	Product profit and royalty			—			11,042			—				11,042
	Net service sales			89,558			—			—				89,558
	Outlicensing revenue			—			1,675			—				1,675
	Revenues from research and development contracts			5,652			5,395			—				11,047
		Total revenues		1,040,839			18,112			—				1,058,951
Operating costs and expenses:
	Cost of products sold			213,543			—			—				213,543
	Cost of services sold			56,287			—			—				56,287
	Selling, general and administrative			296,070			12,604			(4,291	)	[A4]		304,383
	Research and development (including research and development related to contracts)			192,186			37,409			—				229,595
	Amortization of intangibles			53,490			2,492			9,537		[A1]		65,519
		Total operating costs and expenses		811,576			52,505			5,246				869,327
Operating income (loss)				229,263			(34,393	)		(5,246	)			189,624
Other income (expenses):
	Equity in loss of equity method investments			(8,105	)		—			—				(8,105	)
	Minority interest			2,126			—			—				2,126
	Gain on investments in equity securities			424			—			—				424
	Other			(714	)		11			—				(703	)
	Investment income			13,279			1,195			—				14,474
	Interest expense			(27,821	)		(1,027	)		—				(28,848	)
		Total other income (expenses)		(20,811	)		179			—				(20,632	)
Income (loss) before income taxes				208,452			(34,214	)		(5,246	)			168,992
(Provision for) benefit from income taxes				(62,382	)		(15	)		14,536		[A2]		(47,861	)
Net income (loss)			$	146,070		$	(34,229	)	$	9,290			$	121,131

See Notes to Unaudited, Pro Forma Combined Financial Statements.

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GENZYME CORPORATION
UNAUDITED, PRO FORMA COMBINED STATEMENTS OF OPERATIONS
For the Six Months Ended June 30, 2004
(Amounts in thousands, except per share amounts)

			Historical Genzyme Corporation		Historical ILEX Oncology, Inc.			Pro Forma Adjustments		Footnote Reference	Pro Forma Genzyme Corporation
Net income (loss) per share allocated to Genzyme Stock:
	Net income (loss) allocated to Genzyme Stock		$	146,070	$	(34,229	)	$	9,290	[A3]	$	121,131
	Net income (loss) per share of Genzyme Stock:
		Basic	$	0.65				��			$	0.50
		Diluted	$	0.63							$	0.48
	Weighted average shares outstanding:
		Basic		226,145								244,524
		Diluted		231,731								250,395

(1)

Effective July 1, 2003, in connection with the elimination of its tracking stock structure, we ceased allocating earnings to Genzyme Biosurgery and Genzyme Molecular Oncology. From that date forward, all of our earnings are allocated to Genzyme General. Earnings or losses allocated to Genzyme Biosurgery and Genzyme Molecular Oncology prior to July 1, 2003 remain allocated to those divisions and are not affected by the elimination of our tracking stock structure.

(2)

From July 1, 2003 through May 27, 2004, we referred to our outstanding series of common stock as Genzyme General Stock. At our annual meeting of shareholders on May 27, 2004, our shareholders approved an amendment to our charter that eliminated the designation of separate series of common stock, resulting in 690,000,000 authorized shares of a single series of common stock, which we refer to as Genzyme Stock.

See Notes to Unaudited, Pro Forma Combined Financial Statements.

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GENZYME CORPORATION
UNAUDITED, PRO FORMA COMBINED BALANCE SHEETS
As of June 30, 2004
(Amounts in thousands, except par value amounts)

			Historical Genzyme Corporation			Historical ILEX Oncology, Inc.			Pro Forma Adjustments			Footnote Reference	Pro Forma Genzyme Corporation
ASSETS
Current assets:
	Cash and cash equivalents		$	289,857		$	64,906		$	—			$	354,763
	Short-term investments			10,543			59,729			—				70,272
	Restricted short-term investments			—			816			—				816
	Accounts receivable, net			473,053			9,218			—				482,271
	Inventories			268,920			—			—				268,920
	Prepaid expenses and other current assets			69,422			3,966			—				73,388
	Deferred tax assets			133,433			—			39,613		[A5]		173,046
		Total current assets		1,245,228			138,635			39,613				1,423,476
Property, plant and equipment, net				1,202,580			2,910			—				1,205,490
Long-term investments				356,647			18,062			—				374,709
Restricted long-term investments				—			1,691			—				1,691
Notes receivable—related parties				9,470			—			—				9,470
Goodwill, net				803,178			—			298,328		[A5]		1,101,506
Other intangible assets, net				895,237			57,334			231,366		[A5]		1,183,937
Investments in equity securities				121,651			—			—				121,651
Other noncurrent assets				70,764			1,352			(1,266	)	[A6]		70,850
		Total assets	$	4,704,755		$	219,984		$	568,041			$	5,492,780
LIABILITIES AND STOCKHOLDERS' EQUITY
Current liabilities:
	Accounts payable		$	77,543		$	10,195		$	—			$	87,738
	Accrued expenses			262,869			5,841			12,154		[A6]		280,864
	Deferred revenue			12,611			512			—				13,123
	Current portion of long-term debt and capital lease obligations			144,480			19,418			—				163,898
		Total current liabilities		497,503			35,966			12,154				545,623
Long-term debt and capital lease obligations				148,131			—			—				148,131
Convertible notes				690,000			—			—				690,000
Deferred revenue—noncurrent				7,870			902							8,772
Deferred tax liabilities				186,213			—			21,443		[A5]		207,656
Other noncurrent liabilities				51,349			734			—				52,083
		Total liabilities		1,581,066			37,602			33,597				1,652,265
Commitments and contingencies
Stockholders' equity:
	Preferred stock, $0.01 par value			—			—			—				—
	Common stock-Genzyme Stock, $0.01 par value			2,269			—			184		[A5]		2,453
	Additional paid-in capital-Genzyme Stock			3,015,917			—			991,242		[A5]		4,007,159
	Notes receivable from stockholders			(13,557	)		—			—				(13,557	)
	Accumulated deficit			(52,490	)		—			(274,600	)	[A5]		(327,090	)
	Accumulated other comprehensive income			171,550			—			—				171,550
	ILEX Oncology, Inc. convertible preferred stock, $0.01 par value			—			—			—				—
	ILEX Oncology, Inc. common stock, $0.01 par value			—			391			(391	)	[A7]		—
	ILEX Oncology, Inc. additional paid-in capital			—			561,292			(561,292	)	[A7]		—
	ILEX Oncology, Inc. deferred compensation			—			(215	)		215		[A7]		—
	ILEX Oncology, Inc. accumulated deficit			—			(379,773	)		379,773		[A7]		—
	ILEX Oncology, Inc. accumulated other comprehensive income			—			687			(687	)	[A7]		—
		Total stockholders' equity		3,123,689			182,382			534,444				3,840,515
		Total liabilities and stockholders' equity	$	4,704,755		$	219,984		$	568,041			$	5,492,780

See Notes to Unaudited, Pro Forma Combined Financial Statements.

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NOTES TO UNAUDITED, PRO FORMA COMBINED FINANCIAL STATEMENTS

(1) ACCOUNTING POLICIES AND PRO FORMA INFORMATION

Allocation of Earnings and Elimination of the Designation of Separate Series of Common Stock

        Effective July 1, 2003, in connection with the elimination of its tracking stock structure, we ceased allocating earnings to Genzyme Biosurgery and Genzyme Molecular Oncology. From that date forward, all of our earnings are allocated to Genzyme General. Earnings or losses allocated to Genzyme Biosurgery and Genzyme Molecular Oncology prior to July 1, 2003 remain allocated to those divisions and are not affected by the elimination of our tracking stock structure.

        From July 1, 2003 through May 27, 2004, we referred to our outstanding series of common stock as Genzyme General Stock. At our annual meeting of shareholders on May 27, 2004, our shareholders approved an amendment to our charter that eliminated the designation of separate series of common stock, resulting in 690,000,000 authorized shares of a single series of common stock, which we refer to as Genzyme Stock.

Pro Forma Information

        The following unaudited, pro forma combined financial information describes the pro forma effect of our proposed merger with ILEX on our statements of operations for the year ended December 31, 2003 and for the six months ended June 30, 2004, as if the transaction had occurred on January 1, 2003, and our balance sheet as of June 30, 2004, as if the transaction had occurred on this date. Certain historical financial information of ILEX has been reclassified to conform to the presentation of our historical financial statements.

(2) PURCHASE PRICE ALLOCATION

        The estimated aggregate purchase price is comprised of the following (amounts in thousands):

Issuance of 18,379,383 shares of Genzyme Stock	$	946,722
Issuance of options to purchase 1,887,760 shares of Genzyme Stock		44,704
Transaction costs		8,810
Total purchase price	$	1,000,236

        Upon the closing of the transaction (the measurement date), the exchange ratio determining the number of shares of Genzyme Stock to be issued in the transaction will be established. The exact number of shares to be issued may change as a result of changes in the number of shares of ILEX common stock outstanding prior to closing and fluctuations in the price of Genzyme Stock. Assuming that a measurement date occurred on February 26, 2004, the date on which the transaction was announced, approximately 18.4 million shares of Genzyme Stock valued at $946.7 million would have been issued in exchange for the outstanding shares of ILEX common stock, based on the six day average trading price commencing two days before the assumed measurement date.

        Options to purchase approximately 4,010,538 shares of ILEX common stock would be exchanged for options to purchase approximately 1,887,760 shares of Genzyme Stock based on the assumed exchange ratio. Using the assumed value of Genzyme Stock to be issued in the transaction and certain other assumptions in the Black-Scholes option valuation model, the Genzyme options issued in exchange for the ILEX options were valued at approximately $44.7 million. The exact number of options to purchase shares of Genzyme Stock and the total estimated value of these options to be

7

issued may change as a result of changes in the number of options to purchase shares of ILEX common stock outstanding prior to closing and fluctuations in the closing price of Genzyme Stock.

        For purposes of the unaudited, pro forma combined financial statements, we allocated the aggregate purchase price of approximately $1.0 billion to the acquired tangible and intangible assets and assumed liabilities of ILEX based on their estimated respective fair values as of June 30, 2004 (amounts in thousands):

Cash and cash equivalents		$	64,906
Short-term investments			58,674
Restricted short-term investments			816
Accounts receivable			9,218
Deferred tax assets-current			39,613
Other current assets			5,021
Long-term investments			18,062
Restricted long-term investments			1,691
Property, plant and equipment			2,910
Other intangible assets (to be amortized straight-line over 12 years)			288,700
Goodwill			298,328
In-process research and development			274,600
Other assets			1,352
Assumed liabilities:
	Note payable		(19,418	)
	Other assumed liabilities		(22,794	)
Deferred tax liability			(21,443	)
Allocated purchase price		$	1,000,236

        The total purchase price, the estimated fair values of assets acquired and liabilities assumed, the allocation of purchase price and the lives of intangible assets would be determined upon completion of the transaction and may vary from the amounts we have presented in these unaudited, pro forma combined financial statements.

        In connection with the proposed transaction, we would acquire in-process research and development, or IPR&D, related to three development projects, CAMPATH (for indications other than B-cell chronic lymphocytic leukemia), clofarabine and ILX-651.

        CAMPATH is a humanized monoclonal antibody that binds to a specific target, CD52, on cell surfaces leading the body's immune system to destroy malignant, or cancerous, cells. CAMPATH was launched in May 2001 in the United States and in August 2001 in Europe under the name MABCAMPATH. The product is approved for use in patients with B-cell chronic lymphocytic leukemia who have been treated with alkylating agents and who have failed fludarabine therapy, and ILEX is now conducting trials in non-Hodgkin's lymphoma and multiple sclerosis as well.

        Clofarabine is a next-generation, purine nucleoside antimetabolite that is currently under investigation in pediatric and adult leukemias and solid tumors. ILEX has submitted the final section of its "rolling" New Drug Application with the FDA for acute pediatric leukemias based on phase II clinical trial data.

8

        ILX-651 is a next-generation synthetic pentapeptide analog of the natural substance dolastatin-15. This product candidate targets tubulin and has been chemically modified to provide improved pharmacological properties over earlier members of its class. ILEX initiated phase II clinical trials of ILX-651 in late 2003 in recurrent or metastatic melanoma and in locally advanced or metastatic non-small cell lung cancer.

        As of June 30, 2004, none of these projects had reached technological feasibility nor had an alternative future use. Accordingly, we allocated to IPR&D, and charged to accumulated deficit in our unaudited, pro forma combined balance sheet as of June 30, 2004, $274.6 million, representing the portion of the purchase price attributable to these projects, of which $115.8 million is attributable to the CAMPATH development project, $118.6 million is attributable to the clofarabine development project and $40.2 million is related to the ILX-651 development project. The portion of the purchase price estimated to be attributable to these programs would be charged to expense upon completion of the transaction. Material non-recurring charges, such as IPR&D, are not reflected in these unaudited, pro forma combined financial statements of operations for the year ended December 31, 2003 and six months ended June 30, 2004.

        Management assumes responsibility for determining the IPR&D valuation. The fair value assigned to purchased IPR&D was estimated by discounting, to present value, the cash flows expected to result from each project once it has reached technological feasibility. We used a discount rate of 11% and cash flows that have been probability-adjusted to reflect the risks of advancement through the product approval process. In estimating future cash flows, we also considered other tangible and intangible assets required for successful exploitation of the technology resulting from the purchased IPR&D projects and adjusted future cash flows for a charge reflecting the contribution to value of these assets.

(3) PRO FORMA ADJUSTMENTS

        The following adjustments reflect the exchange of ILEX common stock for Genzyme Stock in the transaction as well as the exchange of options to purchase shares of ILEX common stock for fully vested options to purchase shares of Genzyme Stock.

I. Unaudited, Pro Forma Combined Statements of Operations of Genzyme

(A1)

To record the amortization of acquired other intangible assets, all of which will be amortized straight-line over 12 years, for the year ended December 31, 2003 and the six months ended June 30, 2004 (amounts in thousands):

Other Intangible Assets:		Assigned Value		Annual Amortization		Six Months Amortization
Developed technology		$	178,900	$	14,908	$	7,454
Core technology			105,500		8,792		4,396
Trademark			2,400		200		100
Technology license			1,900		158		79
	Total	$	288,700	$	24,058	$	12,029

(A2)

To adjust the tax provision for the impact of the historical losses of ILEX and the pro forma adjustments related to the proposed transaction.

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(A3)

The pro forma adjustments to net income (loss) reflect the aggregate impact of all pro forma adjustments on our income (loss).

(A4)

To eliminate $4.3 million of non-recurring transaction costs included in the historical statements of operations of ILEX for the six months ended June 30, 2004. There were no similar transaction costs included in the historical statements of operations of ILEX for the year ended December 31, 2003.

II. Unaudited, Pro Forma Combined Balance Sheets of Genzyme

(A5)

To record the assumption of the net assets of ILEX and the issuance of approximately 18.4 million shares of Genzyme Stock.

The other intangible assets of approximately $288.7 million are comprised of the following (amounts in thousands):

Other Intangible Assets:		Assigned Value
Developed technology		$	178,900
Core technology			105,500
Trademark			2,400
Technology license			1,900
	Total	$	288,700

Material non-recurring charges, such as the acquired IPR&D charge of $274.6 million resulting from the proposed transaction, are not reflected in our unaudited, pro forma combined statements of operations for the year ended December 31, 2003 or six months ended June 30, 2004. The $274.6 million allocated to IPR&D has been charged to accumulated deficit for purposes of the pro forma balance sheet presentation only and would be charged to expense in our statements of operations upon completion of the transaction.

(A6)

To accrue the following expenses which have not been reflected in our historical balances as of June 30, 2004:

•

$7.5 million for the estimated merger costs we expect to incur; and

•

$4.6 million for severance and bonus payments to certain officers of ILEX that we would be contractually obligated to pay.

Our historical other noncurrent assets as of June 30, 2004 include $1.3 million of merger costs incurred prior to that date.

(A7)

To eliminate ILEX's historical stockholders' equity amounts totaling $182.4 million.

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Exhibit 99.1

UNAUDITED, PRO FORMA COMBINED FINANCIAL INFORMATION