Studies of the adverse effects of occupational exposures are most informative when jobs can be classified according to the specific chemical or physical agents involved. However, information sufficiently detailed to permit this is not often available. In such instances a study usually will be based on subjects classified according to industry or task, effectively ignoring the specific agents to which exposure occurred. Such crude classification tends to reduce or obscure associations between specific exposures and disease. In response to this problem, the authors have developed an occupation and exposure linkage system with particular reference to carcinogenesis. Occupations were classified by industry and by task within the industry. Lists of suspect carcinogens were developed and each suspect carcinogen was linked to industries and tasks in which it has been used. These links make it possible to place in the same exposure category all study subjects whose employment history suggests contact with a particular agent. Epidemiologic analysis then can be based on chemical and physical exposures, rather than on industries or tasks. The linkage system enhances the value of information on occupation obtained from death certificates, cancer registries, medical records, and questionnaires.