Exposure to benzene is thought to be responsible for a higher than normal incidence of leukemia. Benzene is the only industrial chemical associated with the development of human cancer in the lymphoid tissues. Benzidine is an aromatic amine used as an intermediate in the production of dyes and associated with the development of human bladder cancer. The purpose of cancer epidemiology is to identify situations in which there is an increased risk of cancer. Prevention of occupational cancer entails the identification of carcinogenic substances in the workplace and the reduction of human exposure to these substances. Biochemical epidemiology is another approach that is proving useful for identifying individuals at high risk for cancer through exposure to carcinogens in occupational settings. The principal methods for reducing the risk of occupational cancer include the identification of substances in the workplace that induce cancer and the reduction of exposure of workers to these substances.