Commercial city directories, currently produced in 1,250 United States cities, potentially provide yearly information on occupation and employer for all city residents over age 18 years. To investigate the usefulness of these occupational data, the authors have conducted a case-control study of male bladder cancer mortality in Hamilton County, Ohio (which includes Cincinnati). A total of 731 bladder cancer cases who died during 1960-1982 were matched on age, sex, race, date-of-death, and residence at death to two controls per case. Risks of bladder cancer death were calculated by occupation, industry, and specific employer, using both city directories (multiple statements) and death certificates (single statement). Four companies showed a significant excess bladder cancer risk when using city directories. Only one would have been identified using death certificates, which ask for usual lifetime type of industry rather than a specific company name. Using city directories, significant positive associations were found between bladder cancer and occupation as an engineer, tailor, carpenter, furnace operator, blending machine operator, chemist, pressing machine operator, house cleaner, or salesman. For industry, the authors found significant positive associations for the textile, chemical, grain mill, foundry, petroleum, building service, entertainment, and advertising industries. A significant increase in risk for those with 20 or more years of employment was seen for those employed as truck drivers and furnace operators, or those employed in the railroad industry. A check of the validity of city directory data indicated that 77 per cent of the listings agreed with Social Security earnings reports for employer in any given year. One limitation of Hamilton County city directory data was the fairly large number of yearly listings without any occupational data (15 per cent for occupation, 36 per cent for employer). While city directory data do provide work history over time, unlike death certificates, such data are available only for years of residence in the city in question.