-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 JnnuxTfThMS6xklRCE/n67wq7a95l2oQsDHEPfOG8Z++FNOxBaiGKBrqNQrgQneq
 4F91wK+21zoXd5bnsRg+lA==

<SEC-DOCUMENT>0001179110-09-001490.txt : 20090123
<SEC-HEADER>0001179110-09-001490.hdr.sgml : 20090123
<ACCEPTANCE-DATETIME>20090123172514
ACCESSION NUMBER:		0001179110-09-001490
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090121
FILED AS OF DATE:		20090123
DATE AS OF CHANGE:		20090123

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Lane Brian E.
		CENTRAL INDEX KEY:			0001448240

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-13011
		FILM NUMBER:		09543240

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		713.830.9653

	MAIL ADDRESS:	
		STREET 1:		COMFORT SYSTEMS USA, INC.
		STREET 2:		777 POST OAK BLVD., SUITE 500
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77056

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			COMFORT SYSTEMS USA INC
		CENTRAL INDEX KEY:			0001035983
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRICAL WORK [1731]
		IRS NUMBER:				760526487
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		777 POST OAK BOULEVARD
		STREET 2:		SUITE 500
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77056
		BUSINESS PHONE:		7138309600

	MAIL ADDRESS:	
		STREET 1:		777 POST OAK BOULEVARD
		STREET 2:		SUITE 500
		CITY:			HOUSTON
		STATE:			TX
		ZIP:			77056
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-01-21</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001035983</issuerCik>
        <issuerName>COMFORT SYSTEMS USA INC</issuerName>
        <issuerTradingSymbol>FIX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001448240</rptOwnerCik>
            <rptOwnerName>Lane Brian E.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O COMFORT SYSTEMS USA, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>777 POST OAK BLVD., SUITE 500</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77056</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Chief Operating Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2009-01-21</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>827</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>40029</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Price is based on high and low daily stock price average of restricted common stock vesting date (January 21, 2009).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Brian E. Lane</signatureName>
        <signatureDate>2009-01-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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