Mortality patterns up to 1975 were studied in workers occupationally exposed to benzene during 1940-1949 in the production of a natural rubber cast film, rubber hydrochloride, marketed under the trade name Pliofilm, at two localities in Ohio. In comparison with two control populations, a significant excess of leukemia was observed. A five fold excessive risk of all leukemias and a ten-fold excess of deaths from myeloid and monocytic leukemias combined are demonstrated in the study population compared with controls. These figures underestimate the true leukemia risk to benzene (71432) exposed workers, because follow up is only 75 percent complete and the untraced 25 percent of the study population were all regarded, in the statistical analysis, as being alive at the end of the study period. The environment of the workers in the study population was not contaminated with solvents other than benzene, and existing records indicate that the benzene levels themselves were generally below the limits recommended at the time of their measurement.