Objective: The objective of this study was to evaluate work-related asthma among health maintenance organization (HMO) members. Recent reports suggest that the incidence of work-related asthma may be much higher than Sentinel Event Notification Systems for Occupational Risks (SENSOR) data estimate. Methods: Using the HMO's electronic medical record, we identified 1747 persons with evidence of new or recurrent asthma. Interviews with 352 of them elicited information about workplace exposures, symptoms, and home environment. Industrial hygienists rated the potential asthmagenicity of the respondents' work environments. Results: Based on the industrial hygienist ratings and self-reported work-relatedness of asthma symptoms, we classified 33% of those interviewed as having potentially work-related asthma, suggesting an overall work-related asthma incidence/recurrence rate of 28 cases per 10,000. Conclusions: The contribution of occupation to the occurrence of adult onset asthma may be much higher than typically suggested in the literature.