To examine workplace exposure to secondhand smoke by occupation, we analyzed data from The Third National Health and Nutrition Examination Survey (NHANES III) (1988 to 1994), a nationally representative sample of the noninstitutionalized population. The analysis was restricted to 4952 employed nonsmoking adults who reported no home exposure to cigarette smoke. Occupations were assigned to 40 groups and 7 categories. Among the categories, geometric mean serum cotinine (ng/mL) ranged from 0.09 for farming/forestry/fishing occupations to 0.22 for operators/fabricators/laborers (median, 0.16). The lowest values were observed among farmers and nursery workers (0.06) and the highest among waiters (0.47). Between 1988 to 1991 and 1991 to 1994, the overall geometric mean cotinine and the proportion reporting that they could smell smoke at work decreased significantly. In conclusion, workplace exposure to secondhand smoke varied by occupation, and decreases in exposure occurred between 1988 to 1991 and 1991 to 1994.