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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2013-12-10</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001195500</rptOwnerCik>
            <rptOwnerName>CROCKER DOUGLAS II</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>71 SOUTH WACKER DRIVE, SUITE 3400</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>CHICAGO,</rptOwnerCity>
            <rptOwnerState>IL</rptOwnerState>
            <rptOwnerZipCode>60606</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</value>
            </securityTitle>
            <transactionDate>
                <value>2013-12-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>10000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>58.0101</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>93377.139</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">The price reported in column 4 is a weighted average price. These shares were purchased by the Reporting Person in multiple transactions at prices ranging from $57.90 to $58.15, inclusive.  The Reporting Person undertakes to provide Issuer, any security holder of Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote 1.</footnote>
        <footnote id="F2">As of December 10, 2013, Reporting Person also owns options to purchase 25,000 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>Douglas Crocker, II, By: T. Richard Riney, Attorney-In-Fact</signatureName>
        <signatureDate>2013-12-12</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2013-12-10
0000740260
VENTAS INC
VTR
0001195500
CROCKER DOUGLAS II
71 SOUTH WACKER DRIVE, SUITE 3400
CHICAGO,
IL
60606
1
0
0
0
Common Stock
2013-12-10
4
P
0
10000
58.0101
A
93377.139
D
The price reported in column 4 is a weighted average price. These shares were purchased by the Reporting Person in multiple transactions at prices ranging from $57.90 to $58.15, inclusive.  The Reporting Person undertakes to provide Issuer, any security holder of Issuer or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth in this footnote 1.
As of December 10, 2013, Reporting Person also owns options to purchase 25,000 shares of Issuer's common stock.
Douglas Crocker, II, By: T. Richard Riney, Attorney-In-Fact
2013-12-12