We attempted to determine the association between occupational and air pollutant exposure with the development of adult asthma through the analysis of a standardized respiratory questionnaire administered to a cohort of 3914 nonsmoking adults in 1977 and again in 1987. Ambient air pollution concentrations were estimated over a 20-year period using monthly interpolations from fixed-site monitoring stations applied to zip code locations by month of residence and work site. Second-hand smoke exposure was significantly associated with the development of asthma (related risk [RR] = 1.45, confidence interval [CI] = 1.21 to 1.75). Airways obstructive disease before age 16 was related to a marked increased risk (RR = 4.24, CI = 4.03 to 4.45). An increased risk of asthma was significantly associated with increased ambient concentrations of ozone exposure in men (RR = 3.12, CI = 1.61 to 5.85).