Introduction: Thirdhand tobacco smoke consists of substances remaining on the surfaces or in the dust of areas where people have smoked. While previous studies have demonstrated the presence of nicotine and various other constituents of tobacco smoke on surfaces in smokers' homes, none has investigated the presence of tobacco-specific carcinogens. Methods: We used liquid chromatography-tandem mass spectrometry to analyze surface dust samples from both the homes of smokers and nonsmokers for the powerful tobacco-specific lung carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK). Results: We positively identified NNK on surfaces in 33 of 37 smokers' homes (700±788 pg/100cm2 [range, not detected-3,500 pg/100cm2]), but only in 3 of 19 nonsmokers' homes (235±176 pg/100cm2 in the homes where NNK was detected [range, not detected-435 pg/100cm2]). The differences in occurrence and levels of NNK in the homes of smokers and nonsmokers were significant (p <.0001). Conclusions: The powerful tobacco-specific lung carcinogen NNK is present on surfaces in most homes occupied by smokers. Potential renters or buyers of apartments or homes should be notified if previous residents were smokers in order to avoid unnecessary exposure of their families to a potent lung carcinogen.