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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
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 f2JpLuJZldV2K1xh+IKn7w==

<SEC-DOCUMENT>0001231977-09-000033.txt : 20090316
<SEC-HEADER>0001231977-09-000033.hdr.sgml : 20090316
<ACCEPTANCE-DATETIME>20090316143805
ACCESSION NUMBER:		0001231977-09-000033
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090313
FILED AS OF DATE:		20090316
DATE AS OF CHANGE:		20090316

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			KEISER DAVID W
		CENTRAL INDEX KEY:			0001231953

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-27756
		FILM NUMBER:		09683791

	MAIL ADDRESS:	
		STREET 1:		C/O ALEXION PHARMACEUTICALS INC
		STREET 2:		352 KNOTTER DRIVE
		CITY:			CHESHIRE
		STATE:			CT
		ZIP:			06410

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
</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>2009-03-13</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001231953</rptOwnerCik>
            <rptOwnerName>KEISER DAVID W</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>2009-03-13</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10000</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>35.01</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>191060</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The transactions reported by this Form 4 are made pursuant to the terms of a sales plan designed to meet the requirements of Rule 10b5-1 (c)(1) of the Securities Exchange Act.</footnote>
        <footnote id="F2">This transaction was executed in multiple trades through a broker-dealer at prices ranging from $35.00 - $35.99.  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/ David Keiser</signatureName>
        <signatureDate>2009-03-16</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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