Extending a study of the period 1935-1964, we determined the incidence rates of leukemia in Olmsted County during the decade 1965-1974. In this decade, the overall crude annual incidence rate was 9.5 per 100,000. The ratio of males to females was 2.4 to 1; and the incidence rate was significantly higher among males than among females, especially in the older age groups. Adjusted for age and sex, the mean annual incidence rate among males (17.3) was significantly higher than rates reported previously from other areas (ranging from 10.7 to 13); and it had increased significantly during the 40 years 1935-1974 in Olmsted County. The increase was most prominent in males aged 50 and over, and was detectable only in the subgroup with acute myelocytic leukemia. The observed number of farmers (8) was significantly higher than expected (2.8).