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

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2014-09-05</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000104207</issuerCik>
        <issuerName>WALGREEN CO</issuerName>
        <issuerTradingSymbol>WAG</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001040208</rptOwnerCik>
            <rptOwnerName>Rosenstein Barry</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O JANA PARTNERS LLC</rptOwnerStreet1>
            <rptOwnerStreet2>767 FIFTH AVENUE, 8TH FLOOR</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10153</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001159159</rptOwnerCik>
            <rptOwnerName>JANA PARTNERS LLC</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>767 FIFTH AVENUE, 8TH FLOOR</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10153</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, par value $.078125 per share</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12500000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>See footnote</value>
                    <footnoteId id="F1"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">JANA Partners LLC (&quot;JANA&quot;) is a private money management firm which beneficially owns 12,500,000 shares of the Issuer's common stock through various accounts under its management and control. Barry Rosenstein is the Managing Partner of JANA. JANA and Barry Rosenstein disclaim any beneficial ownership of any of the Issuer's securities reported herein except to the extent of their pecuniary interest therein.</footnote>
    </footnotes>

    <remarks>JANA may be deemed to be a director by virtue of the fact that Barry Rosenstein, Managing Partner of JANA, currently serves on the board of directors of the Issuer.</remarks>

    <ownerSignature>
        <signatureName>/s/ Barry Rosenstein______________</signatureName>
        <signatureDate>2014-09-08</signatureDate>
    </ownerSignature>

    <ownerSignature>
        <signatureName>/s/ JANA Partners LLC, by Jennifer Fanjiang, General Counsel</signatureName>
        <signatureDate>2014-09-08</signatureDate>
    </ownerSignature>
</ownershipDocument>
