The relationship between body mass index (BMI) at different ages and subsequent endometrial-cancer risk was investigated in a multicentre case-control study conducted between 1988 and 1991 in Vaud, Switzerland, and Northern Italy on 272 histologically confirmed incident cases of endometrial cancer and 571 controls admitted to hospital for acute, non-neoplastic conditions, unrelated to known or potential risk factors for endometrial cancer. The risk of endometrial cancer increased with increasing BMI in the 3rd decade of age (20 to 29 years), in the 5th decade (40 to 49 years) and in the 7th decade (60 to 69 years), although the risk estimates tended to be substantially higher at older ages: compared with women whose BMI (kg m-2) was less than 20, the relative risks (RR) were 1.8 for BMI greater than or equal to 25 at age 20 to 29, 2.7 for BMI greater than or equal to 30 at age 40 to 49 and 3.8 at age 60 to 69. All the trends in risk were significant, except that for BMI at age 25 after allowance for current BMI. When data were examined in separate strata of current BMI, among women of normal body mass at diagnosis no significant effect of past overweight was observed. In contrast, among subjects over-weight at diagnosis, there were significant direct relationships with BMI at ages 20 to 29 and 40 to 49. To reduce endometrial cancer risk, it is therefore important to avoid obesity in later middle and older age, and the benefit can be even greater for women who were overweight at younger age.