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Industry-based cohort studies require systems for organizing work history data. Although the ultimate goal may be to assess the hazards of specific exposures, classification of the job titles that comprise work histories serves an important descriptive purpose in itself and is often necessary before exposure data can be obtained. A system we have created for organizing jobs in a study of 135,000 workers at five electric power companies highlights conceptual and practical issues in managing work history data for epidemiological studies. Job characteristics including function, location, and authority were used to develop a system of 28 occupational categories. Comprehensibility, flexibility, and efficiency were important criteria in designing the system. Assessment of exposures was an implicit goal; the same categories will define job-exposure matrices for numerous agents. A combination of computer algorithms and expert judgment was used to assign individual job titles to the categories. This system facilitates examining the effects of various agents and controlling for confounding. The 28 categories can be collapsed and regrouped to analyze disease risks in relation to exposures to magnetic fields and other agents; even exposures not previously considered could be brought into the study with this generic system for organizing the electric power industry.