<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0201</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-01-22</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000732485</issuerCik>
        <issuerName>GENZYME CORP</issuerName>
        <issuerTradingSymbol>GENZ</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001021041</rptOwnerCik>
            <rptOwnerName>WIRTH PETER</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>GENZYME CORPORATION</rptOwnerStreet1>
            <rptOwnerStreet2>ONE KENDALL SQUARE</rptOwnerStreet2>
            <rptOwnerCity>CAMBRIDGE</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>021391562</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Executive Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Genzyme General common stock (GENZ)</value>
            </securityTitle>
            <transactionDate>
                <value>2004-01-22</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2004-01-22</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>50000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>14.00</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>57307</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Genzyme General common stock (GENZ)</value>
            </securityTitle>
            <transactionDate>
                <value>2004-01-22</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2004-01-22</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>50000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>56.04</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7307</value>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Genzyme General common stock (GENZ)</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>231</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By 401(k)</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Genzyme General common stock (GENZ)</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>148</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Keogh Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Genzyme General NQ stock option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>14.00</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2004-01-22</value>
            </transactionDate>
            <deemedExecutionDate>
                <value>2004-01-22</value>
            </deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>50000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2003-06-30</value>
            </exerciseDate>
            <expirationDate>
                <value>2006-01-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Genzyme General common stock (GENZ)</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>50000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">The sales reported were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on September 12, 2003.</footnote>
        <footnote id="F2">The exercise of this stock option included a distribution of an additional 12 shares of Genzyme common stock.  These stock options were entitled to receive a distribution of shares of Tissue Repair common stock that was paid on July 22, 1997 to Genzyme General Division stockholders at a rate of 0.015.  In connection with the formation of Genzyme Biosurgery in December 2000, shares of Genzyme Tissue Repair were converted into shares of Genzyme Biosurgery at a rate of 0.3352.  In connection with the exchange of Genzyme Biosurgery stock into Genzyme General stock on June 30, 2003, distributions of the Genzyme Biosurgery stock are exchanged into Genzyme General stock at a rate of 0.04945.</footnote>
        <footnote id="F3">Includes 782 shares purchased from the issuer's Employee Stock Purchase Plan from October 2003 through January 2004.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>By: Susan P. Cogswell, Attorney-in-Fact</signatureName>
        <signatureDate>2004-01-26</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0201
4
2004-01-22
0
0000732485
GENZYME CORP
GENZ
0001021041
WIRTH PETER
GENZYME CORPORATION
ONE KENDALL SQUARE
CAMBRIDGE
MA
021391562
0
1
0
0
Executive Vice President
Genzyme General common stock (GENZ)
2004-01-22
2004-01-22
4
M
0
50000
14.00
A
57307
D
Genzyme General common stock (GENZ)
2004-01-22
2004-01-22
4
S
0
50000
56.04
D
7307
D
Genzyme General common stock (GENZ)
231
I
By 401(k)
Genzyme General common stock (GENZ)
148
I
By Keogh Plan
Genzyme General NQ stock option (right to buy)
14.00
2004-01-22
2004-01-22
4
M
0
50000
0
D
2003-06-30
2006-01-02
Genzyme General common stock (GENZ)
50000
0
D
The sales reported were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on September 12, 2003.
The exercise of this stock option included a distribution of an additional 12 shares of Genzyme common stock.  These stock options were entitled to receive a distribution of shares of Tissue Repair common stock that was paid on July 22, 1997 to Genzyme General Division stockholders at a rate of 0.015.  In connection with the formation of Genzyme Biosurgery in December 2000, shares of Genzyme Tissue Repair were converted into shares of Genzyme Biosurgery at a rate of 0.3352.  In connection with the exchange of Genzyme Biosurgery stock into Genzyme General stock on June 30, 2003, distributions of the Genzyme Biosurgery stock are exchanged into Genzyme General stock at a rate of 0.04945.
Includes 782 shares purchased from the issuer's Employee Stock Purchase Plan from October 2003 through January 2004.
By: Susan P. Cogswell, Attorney-in-Fact
2004-01-26