To estimate the percentage of adult asthmatics with occupational asthma in Michigan, a sample of 94 patients, aged 20 to 65 years, who were discharged from one of three Michigan hospitals in 1990 with the primary diagnosis of asthma were interviewed. The patients were classified using the National Institute for Occupational Safety and Health (NIOSH) criteria as having either probable occupational asthma, possible occupational asthma, or not having occupational asthma. Three percent of the adult asthmatics interviewed met the criteria for having probable occupational asthma. One patient from each hospital had probable occupational asthma. Fifteen of 60 patients (25 percent) from hospital A, 3 of 25 patients (12 percent) from hospital B, and 2 of the 9 patients (22 percent) from hospital C had probable or possible occupational asthma. After adjusting for the difference in response rate among patients from the three hospitals, the percentage of adult asthmatics in Michigan whose asthma is caused by work exposure is estimated to be between 3 percent and 20.2 percent. In contrast, 25.5 percent of the patients stated their asthma was caused by bad working conditions. If these percentages were generalizable to the whole state, then according to our survey 330 to 2,222 or by self-reports of the patients, 2,800 of the 11,000 hospital discharges per year in Michigan for asthma among adults could be attributed to a work exposure.