Occupational asthma (OA) can cause persistent symptoms, but populations with OA have not been followed for the development of serious outcomes such as hospitalization. Subjects receiving compensation for OA during 1980-1993, and a comparison sample of workers with musculoskeletal injuries (INJ) were identified from the Ontario Workers' Compensation Board. We also identified for comparison a group of asthmatic patients (AP) seen at a tertiary care hospital clinic during the same period. The file was matched with the Ontario Ministry of Health data base of hospitalizations through 1996. We compared the frequency of hospitalization of the subgroups with that expected in the general population using standardized morbidity ratios (SMRs), and directly by proportional hazards regression. The study group included 844 OA claimants, 1,556 INJ claimants, and 402 AP. Although admissions for all causes combined and respiratory disease among INJ were less than expected in the general population, admissions for all causes combined exceeded that expected among OA and AP. Admissions for respiratory disease were markedly greater than expected among OA (SMR 9.2) and AP (SMR 17) because of even greater excess admissions for asthma (SMRs 45 and 81, respectively). Compared with those with INJ, those with OA were more likely to be hospitalized for all causes combined (adjusted relative risk [RR] 1.4, 95% confidence interval [CI] 1.2 to 16); cardiovascular disease (RR 1.4, 95% CI 0.9 to 2.0); respiratory disease (RR 5.4, 95% CI 3.8 to 7.7); and asthma (RR 28.1, 95% CI 10.2 to 77.2) but not for malignancies (RR 1.0) or injuries (RR 0.9). Those with OA were admitted to hospital about half as frequently as AP for respiratory disease and asthma (although this was modified by smoking status and sex), but were 30% more likely to be admitted for ischemic heart disease (IHD). Among the OA claimants, factors that were significantly associated with hospitalization for asthma included older age and exposure to agents other than isocyanates. Those with OA became less likely to be hospitalized for asthma with increasing time after onset, particularly after 5 or more years. We conclude that subjects with OA suffer higher rates of hospitalizations for all causes combined, respiratory disease, and asthma than other workers, although less than among AP seen at a tertiary care center.