OBJECTIVES: Occupational health studies often rely on self-reported secondhand smoke (SHS) exposure. This study examines the accuracy of self-reported tobacco use and SHS exposure. METHODS: Data on serum cotinine, self-reported tobacco use, and SHS exposure for US workers were extracted from three National Health and Nutrition Examination Surveys (n = 17,011). Serum cotinine levels were used to classify workers into SHS exposure categories. The percent agreement between self-reported tobacco use and SHS exposure with the cotinine categories was calculated. RESULTS: Workers reporting tobacco use were 88% accurate whereas workers reporting work, home, or home+work exposures were 87% to 92% accurate. Workers reporting no SHS exposure were only 28% accurate. CONCLUSIONS: Workers accurately reported their smoking status and workplace-home SHS exposures, but substantial numbers reporting "no exposures" had detectable levels of cotinine in their blood, indicating exposure to SHS.