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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 KYSKhOq19MBpqJcZVNy0rUVkhtJa+tyewZJyFQfULU56zdXevkTEswiLTnJUDfer
 qAJa42BdalqDwOO32+B0NA==

<SEC-DOCUMENT>0001231977-10-000031.txt : 20100223
<SEC-HEADER>0001231977-10-000031.hdr.sgml : 20100223
<ACCEPTANCE-DATETIME>20100223111924
ACCESSION NUMBER:		0001231977-10-000031
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100223
FILED AS OF DATE:		20100223
DATE AS OF CHANGE:		20100223

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			ALEXION PHARMACEUTICALS INC
		CENTRAL INDEX KEY:			0000899866
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				133648318
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		352 KNOTTER DRIVE
		CITY:			CHESHIRE
		STATE:			CT
		ZIP:			06511
		BUSINESS PHONE:		2037761790

	MAIL ADDRESS:	
		STREET 1:		352 KNOTTER DRIVE
		CITY:			CHESHIRE
		STATE:			CT
		ZIP:			06511

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			LINK MAX
		CENTRAL INDEX KEY:			0001050944

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-27756
		FILM NUMBER:		10624800

	MAIL ADDRESS:	
		STREET 2:		509 MADISON AVE 14TH FL
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10022
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-02-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000899866</issuerCik>
        <issuerName>ALEXION PHARMACEUTICALS INC</issuerName>
        <issuerTradingSymbol>ALXN</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001050944</rptOwnerCik>
            <rptOwnerName>LINK MAX</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ALEXION PHARMACEUTICALS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>352 KNOTTER DRIVE</rptOwnerStreet2>
            <rptOwnerCity>CHESHIRE</rptOwnerCity>
            <rptOwnerState>CT</rptOwnerState>
            <rptOwnerZipCode>06410</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock, par value $.0001 per share</value>
            </securityTitle>
            <transactionDate>
                <value>2010-02-18</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>40000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>49.34</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>122540</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">This transaction was executed in multiple trades through a broker-dealer at prices ranging from $48.61 - $49.61.  The price reported in this column reflects the weighted average sales price.  Upon request, the reporting person will provide to the SEC staff full information regarding the number of shares sold at each price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Max Link</signatureName>
        <signatureDate>2010-02-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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