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

    <schemaVersion>X0304</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-02-27</periodOfReport>

    <issuer>
        <issuerCik>0000740260</issuerCik>
        <issuerName>VENTAS INC</issuerName>
        <issuerTradingSymbol>VTR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001215188</rptOwnerCik>
            <rptOwnerName>CAFARO DEBRA A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>353 N. CLARK STREET, SUITE 3300</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60654</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chairman and CEO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-02-27</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>54822</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>55.1133</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>704030</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5000</value>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On February 27, 2012, the Reporting Person transmitted to the Securities and Exchange Commission (the &quot;SEC&quot;) a Form 144 covering the sale of the Issuer's common stock reported on Table I.  The price reported in column 4 is a weighted average price.  These shares were sold by the Reporting Person in multiple transactions at prices ranging from $55.00 to $55.36, inclusive.  Reporting Person undertakes to provide the Issuer, any security holder of the Issuer or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (1).</footnote>
        <footnote id="F2">These shares are being sold pursuant to a written non-discretionary Rule 10b5-1(c) sales plan dated May 25, 2011.</footnote>
        <footnote id="F3">Reflects shares held by the Cafaro Family Trust, of which Reporting Person's spouse is the trustee.  Reporting Person disclaims beneficial ownership of these 5,000 shares except to the extent of the Reporting Person's pecuniary interest in the shares.</footnote>
        <footnote id="F4">As of February 27, 2012, Reporting Person also owns directly and indirectly by Trust options to purchase 1,186,923 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Debra A. Cafaro, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2012-02-29</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0304
4
2012-02-27
0000740260
VENTAS INC
VTR
0001215188
CAFARO DEBRA A
353 N. CLARK STREET, SUITE 3300
CHICAGO,
IL
60654
1
1
0
0
Chairman and CEO
Common Stock
2012-02-27
4
S
0
54822
55.1133
D
704030
D
Common Stock
5000
I
By Trust
On February 27, 2012, the Reporting Person transmitted to the Securities and Exchange Commission (the "SEC") a Form 144 covering the sale of the Issuer's common stock reported on Table I.  The price reported in column 4 is a weighted average price.  These shares were sold by the Reporting Person in multiple transactions at prices ranging from $55.00 to $55.36, inclusive.  Reporting Person undertakes to provide the Issuer, any security holder of the Issuer or the staff of the SEC, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote (1).
These shares are being sold pursuant to a written non-discretionary Rule 10b5-1(c) sales plan dated May 25, 2011.
Reflects shares held by the Cafaro Family Trust, of which Reporting Person's spouse is the trustee.  Reporting Person disclaims beneficial ownership of these 5,000 shares except to the extent of the Reporting Person's pecuniary interest in the shares.
As of February 27, 2012, Reporting Person also owns directly and indirectly by Trust options to purchase 1,186,923 shares of Issuer's common stock.
Debra A. Cafaro, By: T. Richard Riney, Attorney-In-Fact
2012-02-29