QuickLinks -- Click here to rapidly navigate through this document

Exhibit 5

[ROPES & GRAY LLP Letterhead]

June 18, 2004

Genzyme Corporation
Genzyme Center
500 Kendall Street
Cambridge, Massachusetts 02142

Ladies and Gentlemen:

        This opinion is furnished to you in connection with a registration statement on Form S-8 (the "Registration Statement"), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended, for the registration of 6,800,000 shares of Genzyme Common Stock, $0.01 par value (the "Shares"), of Genzyme Corporation, a Massachusetts corporation (the "Company"). The Shares are issuable under the Company's 2004 Equity Incentive Plan (the "Plan").

        We are familiar with the actions taken by the Company in connection with the proposed issuance of the Shares. For purposes of our opinion, we have examined and relied upon such documents, records, certificates, and other instruments as we have deemed necessary. The opinions expressed below are limited to the laws of the Commonwealth of Massachusetts.

        Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when the Shares have been issued and sold in accordance with the terms of the Plan, the Shares will be validly issued, fully paid, and nonassessable.

        We hereby consent to the filing of this opinion as an exhibit to the Registration Statement. Our consent shall not be deemed an admission that we are experts whose consent is required under Section 7 of the Securities Act of 1933, as amended.

        It is understood that this opinion is to be used only in connection with the offer and sale of Shares while the Registration Statement is in effect.

Very truly yours,

/s/ Ropes & Gray LLP

Ropes & Gray LLP

QuickLinks

[ROPES & GRAY LLP Letterhead]