Biologic markers have provided a direct method for assessing exposure to environmental tobacco smoke (ETS) and yet few studies have used these techniques to document exposure in general samples of nonsmokers. Exposure to ETS was assessed by serum cotinine and self-report in 3300 nonsmoking participants in the CARDIA study. Nonsmoking status was validated by a cotinine level of < 14 ng/ml. Twenty-eight percent of the 18- to 30-year-olds were exposed to ETS as determined by a detectable serum cotinine level (2-13 ng/ml); prevalence of exposure was higher among blacks than whites (32% vs 24%, P < 0.001). Similarly, ETS exposure as defined by self-report (hours/week) was higher in blacks, particularly for exposure in the home and in other small areas. Multivariate predictors of cotinine-determined exposure included reported exposure, male gender, lower education, past smoking history, and spending time with smokers. Only among current users of marijuana, 20% of the sample, was the black race found to be an independent predictor of exposure. The prevalence of ETS exposure is higher in blacks than whites, as documented by self-report and confirmed by serum cotinine levels. Other correlates of exposure include demographic factors and factors which may be surrogate measures of exposure.