Methodological problems in the epidemiologic study of work-related diseases are addressed. Because the occupational etiologic fraction of work-related diseases is small, its demonstration is often difficult. Especially cross-sectional studies are hampered by diluting effects, such as selection out of an exposed job. The problems involved in the study of four disease groups, ie, chronic nonspecific respiratory disease, musculoskeletal disorders, cardiovascular diseases, and behavioral responses and psychosomatic symptomatology are discussed in some detail.