Between 1955 and 1974, 138 cases of leukaemia occurred in residents of Olmsted County, Minnesota. Each case was compared with 2 control subjects matched for age, sex, and residence with regard to their occupational and chemical exposures before the diagnosis of leukaemia. Farming and health-related occupations have been previosly implicated in the development of leukaemia and occurred frequently enough in our group of patients and control subjects to permit analysis. Neither of these 2 occupations occurred more frequently enough in our group of patients and control subjects to permit analysis. Neither of these 2 occupations occurred more frequently in cases than in control subjects.