Occupational asthma may account for a significant proportion of adult-onset asthma, but incidence estimates from surveillance of physician reports and workers' compensation data (0.9 to 15/100,000) are lower than expected from community-based cross-sectional studies of asthma patients. We conducted a prospective cohort study of 79,204 health maintenance organization members between the ages of 15 and 55 at risk for asthma. Computerized files, medical records, and telephone interviews were used to identify and characterize asthma cases. Evidence for asthma attributable to occupational exposure was determined from work-related symptoms and workplace exposure. The annual incidence of clinically significant, new-onset asthma was 1.3/1,000, and increased to 3.7/1,000 when cases with reactivation of previously quiescent asthma were included. Criteria for onset of clinically significant asthma attributable to occupational exposure were met by 21% (95% CI 12-32%) of cases giving an incidence of 71/100,000 (95% CI 43-111). Physicians documented asking about work-related symptoms in 15% of charts, and recorded suggestive symptoms in three cases, but did not obtain occupational medicine consultation, diagnose occupational asthma, report to the state surveillance program, or bill workers' compensation for any of them. These data suggest that the incidence of asthma attributable to occupational exposures is significantly higher than previously reported, and accounts for a sizable proportion of adult-onset asthma.