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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-03-15</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>2013-03-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>573194</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-03-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>G</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>10000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2013-03-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F2"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>70.75</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <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>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents a bona fide gift from Reporting Person to the Cafaro-Livingston Charitable Trust, a private charitable foundation (the &quot;Trust&quot;). Reporting Person is a co-trustee of the Trust and may be deemed to have shared voting and dispositive power over shares held by the Trust. Reporting Person disclaims beneficial ownership of these 5,000 shares because the Reporting Person has no pecuniary interest in the shares.</footnote>
        <footnote id="F2">On March 15, 2013, the Trust transmitted to the Securities and Exchange Commission a Form 144 covering the sale of the Issuer's common stock reported in Table I.</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 March 15, 2013, Reporting Person also owns directly and indirectly by trust options to purchase 1,237,128 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Debra A. Cafaro, By T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-03-19</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-03-15
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
2013-03-15
5
G
0
5000
0
D
573194
D
Common Stock
2013-03-15
5
G
0
5000
0
A
10000
I
By Trust
Common Stock
2013-03-15
4
S
0
5000
70.75
D
5000
I
By Trust
Represents a bona fide gift from Reporting Person to the Cafaro-Livingston Charitable Trust, a private charitable foundation (the "Trust"). Reporting Person is a co-trustee of the Trust and may be deemed to have shared voting and dispositive power over shares held by the Trust. Reporting Person disclaims beneficial ownership of these 5,000 shares because the Reporting Person has no pecuniary interest in the shares.
On March 15, 2013, the Trust transmitted to the Securities and Exchange Commission a Form 144 covering the sale of the Issuer's common stock reported in Table I.
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 March 15, 2013, Reporting Person also owns directly and indirectly by trust options to purchase 1,237,128 shares of Issuer's common stock.
Debra A. Cafaro, By T. Richard Riney, Attorney-In-Fact
2013-03-19