Objectives. To assess the proportion of adult asthma at the state level that may be related to work. Design. Work-related asthma questions were added to the 2001 Behavioral Risk Factor Surveillance System (BRFSS) questionnaire in California, Massachusetts, and Michigan. Results. Findings indicate 7.4-9.7% of those with current asthma reported that their asthma may be work related. These results estimate that approximately 137,000 adults in California, 39,000 in Massachusetts, and 63,000 in Michigan have asthma that may be work related. Conclusions. These findings are unique in providing population-based estimates at the state level that illustrate that a substantial portion of adult asthma morbidity is due to exposures in the work environment.