A cohort study has been carried out to investigate risk factors for cancer as well as hyperplasia of the endometrium. Over the 13 years for which we followed 25,000 women aged 40–65 (who took part in a population-based screening programme for breast cancer), 111 cases of endometrial cancer and 109 cases of endometrial hyperplasia were diagnosed. A comparison of the outcome between the two disease entities revealed that large body weight among postmenopausal women and the use of oestrogenic drugs at all ages were risk factors for both cancer and hyperplasia of the endometrium. However, reproductive histories and premenopausal steroid profiles differed. Steroid excretion determinations in urine samples collected years before diagnosis provided further evidence in favour of the hypothesis of unopposed action of oestrogens in the aetiology of endometrial cancer. In women who were to develop endometrial hyperplasia or cancer the obesity–oestrogen relationship was stronger than in those who remained free of endometrial disease during the period of follow-up. The possible significance of differences in aromatase activity among the obese is considered.