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                                  UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D. C. 20549

                                    FORM 8-K

                 CURRENT REPORT PURSUANT TO SECTION 13 OR 15(d)
                     OF THE SECURITIES EXCHANGE ACT OF 1934

                DATE OF REPORT (DATE OF EARLIEST EVENT REPORTED):
                                FEBRUARY 26, 2004

                               GENZYME CORPORATION
             (Exact name of registrant as specified in its charter)

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         MASSACHUSETTS                         0-14680                            06-1047163
(State or other jurisdiction of        (Commission file number)          (IRS employer identification
 incorporation or organization)                                                     number)
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               500 KENDALL STREET, CAMBRIDGE, MASSACHUSETTS 02142
               (Address of Principal Executive Offices) (Zip Code)

               Registrant's telephone number, including area code:
                                 (617) 252-7500

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ITEM 5. OTHER EVENTS

        On February 26, 2004, Genzyme Corporation ("Genzyme"), a Massachusetts
corporation, GLBC Corp. ("Sub"), a Delaware corporation and a wholly-owned
subsidiary of Genzyme, GLBC LLC ("LLC"), a Delaware limited liability company
and a wholly-owned subsidiary of Genzyme, and ILEX Oncology, Inc. ("ILEX"), a
Delaware corporation, entered into an Agreement and Plan of Merger (the "Merger
Agreement") pursuant to which the parties will effect a business combination
through an initial merger of Sub with and into ILEX, and a subsequent merger of
ILEX with and into LLC (the "Mergers"). As a result of the Mergers, ILEX will
become a wholly-owned subsidiary of Genzyme. The business combination is
expected to be accounted for using the purchase method of accounting and to
qualify as a tax-free reorganization within the meaning of Section 368 of the
Internal Revenue Code of 1986.

        Under the terms of the Merger Agreement, each outstanding share of ILEX
common stock, $0.01 par value per share, will be converted into the right to
receive a fraction of a share (the "Exchange Ratio") of Genzyme common stock.
The Exchange Ratio will be calculated by dividing $26.00 by the average per
share closing price of Genzyme common stock (the "Genzyme Share Price") as
reported by the NASDAQ National Market over the twenty trading days ending on
the fifth trading day prior to the closing date of the Mergers, except that if
the Genzyme Share Price is greater than $59.88, the Exchange Ratio will be
0.4342, and if the Genzyme Share Price is less than $46.58, the Exchange Ratio
will be 0.5582. In addition, each outstanding option to purchase ILEX common
stock will be converted into an option to purchase the number of shares of
Genzyme common stock equal to the number of shares of ILEX common stock subject
to such option multiplied by the Exchange Ratio, and the associated exercise
price will be adjusted accordingly.

        Consummation of the Mergers is subject to the adoption of the Merger
Agreement by the ILEX stockholders and certain other customary closing
conditions.

        The preceding description of the Merger Agreement and the transactions
contemplated thereby is qualified in its entirety by reference to the copy of
the Merger Agreement included as Exhibit 2.1 hereto and incorporated by
reference herein.

        A joint press release issued by Genzyme and ILEX announcing the
execution of the Merger Agreement is attached as Exhibit 99.1 hereto.

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                                    SIGNATURE

        Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized.

                                      GENZYME CORPORATION

Dated: February 26, 2004              By: /s/ Michael S. Wyzga
                                         ---------------------------------------
                                          Michael S. Wyzga
                                          Executive Vice President, Finance and
                                          Chief Financial Officer

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                                  EXHIBIT INDEX

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<Caption>
EXHIBIT NO.    DESCRIPTION
- -----------    -----------
            
    2.1        Agreement and Plan of Merger dated as of February 26, 2004 among
               Genzyme Corporation, GLBC Corp., GLBC LLC and ILEX Oncology,
               Inc., filed herewith.

    99.1       Joint Press Release dated February 26, 2004, announcing the
               execution of the Merger Agreement.
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