Data from previous studies suggest that infertility is a risk factor for endometrial cancer. We used data from the Cancer and Steroid Hormone Study to further characterize this relationship. The subject group comprised 399 women ages 20-54 with newly diagnosed epithelial endometrial cancer ascertained through six cancer registries. The control group comprised 3040 women in the same age range selected by random-digit telephone dialing from the same geographic areas where cancer patients resided. Compared with women who reported no fertility problem, women with physician-diagnosed infertility who had reported at least 2 years of infertility had an odds ratio for endometrial cancer, adjusted for age, of 1.7 (95% confidence interval 1.1-2.6). Women who reported infertility resulting from ovarian factors had an adjusted odds ratio of 4.2 (95% confidence interval 1.7-10.4). These results suggest that factors such as anovulation may explain much of the increased risk of endometrial cancer found among subgroups of infertile women.