Reports of recent exposure to environmental tobacco smoke (ETS) and urinary cotinine levels were obtained on 663 never- and ex-smokers who attended a cancer screening clinic in Buffalo, New York, in 1986. Study objectives included determining the prevalence of exposure to ETS using urinary cotinine and identifying questionnaire exposure measures predictive of cotinine. Findings demonstrate that exposure to environmental tobacco smoke is extremely prevalent, even among those not living with a smoker. A total of 76% of subjects reported exposure to ETS in the 4 d preceding the interview. The most frequently mentioned sources of exposure were at work (28%) and at home (27%). Cotinine was found in the urine of 91% of subjects. Cotinine values increased significantly with the number of exposures reported. Among the different questionnaire measures of exposure that were evaluated, the single best predictor of cotinine was the number of friends and family members seen regularly by the subject who smoke.