This review discusses occupational asthma caused by substances encountered in the workplace. The topics discussed include the epidemiology, risk factors, causes, pathogenesis, diagnosis, outcome, therapy, and prevention of occupational asthma. Over 200 organic and inorganic compounds are known to cause occupational asthma, including animal products, insects, birds, plants, biologic enzymes, vegetables, diisocyanates, anhydrides, wood dusts, metals, fluxes, drugs and other chemicals. This list is expected to continue to grow as new materials are introduced into industry. The occupations in which occupational asthma has been identified include laboratory workers, veterinarians, pigeon breeders, grain workers, entomologists, bakers, millers, food processors, pharmaceutical workers, printers, chemists, foundry workers, construction workers, cabinet makers, and metal workers. Occupational epidemiological studies require the cooperation of management, labor and government regulatory agencies. Currently the techniques available to identify subjects with asthma in epidemiological studies are not satisfactory. The methods used to confirm the diagnosis of occupational asthma are also unsatisfactory. Criteria for assessment of functional impairment caused by occupational asthma must be established. Research efforts should be directed toward understanding what constituents impairment and disability in asthma.