The contract will enable the conduct of a national cohort study of persons heavily exposed to benzene, including persons with benzene poisoning. Approximately 100,000 persons with varying levels of exposure to benzene will be identified and their cancer experience during 197287 ascertained. Special attention will be paid to evaluating risks of leukemia, greatly increasing the number of patients available to evaluate doserelated trends. The study will also provide the first largescale assessment of risks of other cancers associated with benzene.