A study at two rubber factories of mortality of workers exposed to benzene (71432) was conducted. The cohort consisted of 1,006 white male workers who had been exposed to benzene vapor. The cohort was subdivided into two groups, one consisting of 748 workers who worked at least 1 day between January 1, 1940 and December 3, 1949 in a department with benzene exposure; and the other consisting of 258 subjects who first worked in a department with benzene exposure between January 1, 1950 and December 31, 1959. The vital status of the cohort was determined as of June 30, 1975. Death certificates for all known deaths were examined. Standardized mortality ratios (SMRs) were calculated. Among the cohort that worked at least 1 day in a benzene exposed department at either facility between 1940 and 1949, overall mortality was 180 deaths, versus 161 expected. There was a significant excess of malignancies of the lymphatic and hematopoietic system, 10 cases observed versus 1.25 expected. The SMRs for leukemia increased with years of exposure. For subjects with at least 5 years exposure to benzene, the SMR for leukemia was 2,100. For subjects with more than 10 years exposure, the SMR was 3,300. Among the subjects who first worked in a benzene exposed department between 1950 and 1959, overall mortality was 49 versus 56 deaths expected. One case of leukemia was observed versus 0.46 expected. The author concludes that there is a strong association between occupational exposure to benzene and deaths from leukemia. He estimates that in a worker population exposed for a working lifetime to 100 parts per million benzene vapor, 140 excess leukemia deaths would occur.