The risk of endometrial cancer (EC) in diabetics and hypertensives was studied as part of the planning of a trial of the efficacy of screening for EC. Two nationwide registers, the Finnish Cancer Registry and the Social Insurance Institution's (SII) population register, were linked to obtain 1715 EC patients diagnosed during 1970-1974 with individually matched controls. Data on the right for state-reimbursed medication for diabetes or hypertension prior to the case's cancer diagnosis was obtained from the SII's population register and was used as indicator of the diseases in question. Diabetes was a strong and hypertension a relatively weak risk factor for EC. The relative risk for the association of EC with diabetes was 4.1 (95% confidence limits 2.7 and 6.9) and with hypertension 1.6 (95% confidence limits 1.3 and 2.1). Diabetics and hypertensives are identifiable risk groups for EC in Finland. Diabetics form a suitable target population for a trial of the efficacy of screening for EC.