With data from a study of 296 patients with primary epithelial ovarian cancer and 343 patients hospitalized because of other conditions, we estimated ovarian cancer risk in accordance with reproductive and other factors. Risk was greatest among women of lower parity, especially among women who said they planned to have children but could not. The protective effect of oral contraceptives seen in other studies was observed only in subgroups of our study population. Women who had breastfed their children had decreased risk, but the number of months of breastfeeding was not related to risk. Incomplete pregnancies did not provide the protection seen for live births. A family history of ovarian cancer and a medical history of breast cancer were both strong risk factors. None of the nonreproductive factors that we examined, including childhood illnesses, tobacco and alcohol consumption, obesity, and selected adult diseases, was convincingly associated with risk.