Information gathered through epidemiologic studies of occupational asthma is useful for the establishment of prevalence or incidence data for public health purposes, which can be used to determine which industry or workplace requires assistance from preventive services. These studies address etiologic questions regarding exposure and host risk factors. They can establish an exposure-response relationship that provides scientific basis for setting environmental control measures. They also examine the effectiveness of preventive 5 control measures. In the conduct of epidemiologic studies of occupational asthma, choosing the type of study to answer the study question is important. Observational studies have predominated in studies of occupational asthma. Although these types of studies have provided much useful data, they are associated with the problem of the healthy-worker effect. In the future, use of more robust epidemiologic study designs should foster a greater understanding of occupational asthma.