During a 15-year period in an urban area with over a million inhabitants, we identified 1,678 cases of leukemia among residents aged 16 or older. Case finding was done according to the methods of the National Cancer Institute (NCI) SEER program. Usual occupation was coded by a system based on the U.S. Census Bureau classification. According to age-standardized incidence rates for persons aged 16-67, significant excess leukemia risks were found for 14 male and 8 female occupation categories, with the larger number of male excess risk situations due to nonlymphatic leukemia. Lymphatic leukemia risks were significantly elevated among dentists, school teachers of both sexes, auto mechanics, gas station attendants, female assembly workers, and female laundry and dry cleaning workers. Nonlymphatic leukemia risks were significantly elevated among machinists, other metal tradesmen, heavy equipment operators, textile operatives, meat cutters, cannery workers, construction laborers, freight and stock handlers, policemen, and firemen. Risks of both types of leukemia were significantly elevated among registered and practical nurses and lumber mill workers. This study has not identified specific etiologic agents and exposures, but applied investigations aimed at disease control by prevention of cases are now possible in this community. Nationwide surveillance and control are recommended.