This is a report on the relationship of combined oral contraceptives to risk of endometrial cancer from a multinational hospital-based case-control study of various steroid contraceptives and five different cancers. Based on data on 130 cases and 835 matched controls from nine participating centres in seven developing countries and two participating centres in two developed countries, the relative risk of endometrial carcinoma in women who ever took combined oral contraceptives was estimated to be 0.55. This value is similar to estimates from prior investigations in the US and Italy, and the protective effect observed in this study was at least as great in the developing countries with low incidence rates as in the developed countries with higher rates. The reduction in risk was observed for adenocarcinomas with and without squamous elements.