-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 SiDwaGU5j0+szUpHdTzNd7f0tliwt7l048W1QBJ9nsJB3uiso2HeeWspda55nIwt
 UQpFeMNDbIJO15q76rQn4Q==

<SEC-DOCUMENT>0001231977-03-000001.txt : 20030611
<SEC-HEADER>0001231977-03-000001.hdr.sgml : 20030611
<ACCEPTANCE-DATETIME>20030611121153
ACCESSION NUMBER:		0001231977-03-000001
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20030611
FILED AS OF DATE:		20030611

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ROLLINS SCOTT A
		CENTRAL INDEX KEY:			0001231952

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-27756
		FILM NUMBER:		03740096

	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:			0731

	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>

    <documentType>4</documentType>

    <periodOfReport>2003-06-11</periodOfReport>


    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001231952</rptOwnerCik>
            <rptOwnerName>ROLLINS SCOTT A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Senior Vice President</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeSecurity>
        <securityTitle>
            <value>Common Stock, par value $.0001</value>
        </securityTitle>
        <transactionDate>
            <value>2003-06-11</value>
        </transactionDate>
        <transactionCoding>
            <transactionFormType>4</transactionFormType>
            <transactionCode>S</transactionCode>
            <equitySwapInvolved>0</equitySwapInvolved>
        </transactionCoding>
        <transactionTimeliness>
            <value></value>
        </transactionTimeliness>
        <transactionAmounts>
            <transactionShares>
                <value>2000</value>
            </transactionShares>
            <transactionValue>
                <value>18.25</value>
            </transactionValue>
            <transactionAcquiredDisposedCode>
                <value>D</value>
            </transactionAcquiredDisposedCode>
        </transactionAmounts>
        <postTransactionAmounts>
            <sharesOwnedFollowingTransaction>
                <value>2000</value>
            </sharesOwnedFollowingTransaction>
        </postTransactionAmounts>
        <ownershipNature>
            <directOrIndirectOwnership>
                <value>D</value>
            </directOrIndirectOwnership>
        </ownershipNature>
    </nonDerivativeSecurity>


    <ownerSignature>
        <signatureName>/s/ Scott A. Rollins</signatureName>
        <signatureDate>2003-06-11</signatureDate>
    </ownerSignature>

</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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