The role of smoking and air pollution in bronchial asthma in otherwise healthy adults is still unclear. We compared 79 cases of asthma, diagnosed between ages 20 and 65 years, with 304 randomly drawn population controls of similar age from the same catchment area as the cases. The comparison involved questionnaire information on smoking habits, occupational exposures, dwelling conditions, various suspect allergenic exposures, and atopy. Those who had smoked for 3 years or more, present or past, were at increased risk for bronchial asthma (odds ratio = 1.9; 95% confidence interval = 1.1-3.3). Adjustment by multiple logistic regression for age and gender as well as atopy and air pollution at work did not change the relative risk estimate. Exposure to environmental tobacco smoke, or passive smoking, at work involved a slightly greater risk.