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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2012-11-12</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001215185</rptOwnerCik>
            <rptOwnerName>RINEY T RICHARD</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>10350 ORMSBY PARK PLACE, SUITE 300</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>LOUISVILLE</rptOwnerCity>
            <rptOwnerState>KY</rptOwnerState>
            <rptOwnerZipCode>40223</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>EVP, Chief Admin. Off. &amp; GC</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2012-11-12</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>52535</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>63.7755</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12465</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Trust</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>70000</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Spouse</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1300</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By IRA</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>109559</value>
                    <footnoteId id="F3"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">On November 12, 2012, the Riney Family Trust (the &quot;Trust&quot;) (of which Reporting Person's spouse is the trustee) transmitted to the Securities and Exchange Commission (the &quot;SEC&quot;) a Form 144 covering the sale of the Issuer's common stock reported in Table I. The shares were sold by the Trust for tax and estate planning purposes. The price reported in column 4 is a weighted average price. These shares were sold by the Trust in multiple transactions at prices ranging from $63.415 to $63.920, inclusive.  The Trust undertakes to provide Issuer, any security holder of 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">Reporting Person disclaims beneficial ownership of these 12,465 shares except to the extent of the Reporting Person's pecuniary interest in the shares.</footnote>
        <footnote id="F3">As of November 12, 2012, Reporting Person also owns options to purchase 71,054 shares of Issuer's common stock.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>T. Richard Riney</signatureName>
        <signatureDate>2012-11-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
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X0306
4
2012-11-12
0000740260
VENTAS INC
VTR
0001215185
RINEY T RICHARD
10350 ORMSBY PARK PLACE, SUITE 300
LOUISVILLE
KY
40223
0
1
0
0
EVP, Chief Admin. Off. & GC
Common Stock
2012-11-12
4
S
0
52535
63.7755
D
12465
I
By Trust
Common Stock
70000
I
By Spouse
Common Stock
1300
I
By IRA
Common Stock
109559
D
On November 12, 2012, the Riney Family Trust (the "Trust") (of which Reporting Person's spouse is the trustee) transmitted to the Securities and Exchange Commission (the "SEC") a Form 144 covering the sale of the Issuer's common stock reported in Table I. The shares were sold by the Trust for tax and estate planning purposes. The price reported in column 4 is a weighted average price. These shares were sold by the Trust in multiple transactions at prices ranging from $63.415 to $63.920, inclusive.  The Trust undertakes to provide Issuer, any security holder of 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).
Reporting Person disclaims beneficial ownership of these 12,465 shares except to the extent of the Reporting Person's pecuniary interest in the shares.
As of November 12, 2012, Reporting Person also owns options to purchase 71,054 shares of Issuer's common stock.
T. Richard Riney
2012-11-14