PURPOSE: The incidence of synchronous primary endometrial and ovarian cancer is 2- to 10-fold higher than that expected based on the incidence of each cancer alone. We sought to evaluate reasons for this in a case-control study. METHODS: We combined data on a maternal history of cancer and reproductive and menstrual factors from 56 women with synchronous multiple primary disease who had participated in three population-based studies of gynecologic cancer. For comparison, we analyzed the same information from 280 women with endometrial cancer alone, 280 with ovarian cancer alone, and 280 without a history of either cancer. RESULTS: The reduced risk of multiple primary disease associated with high parity (2 or more births vs 0: OR = 0.37, 95% Cl, 0.19-76) and long-term use of oral contraceptives (12 or more months vs none: OR = 0.60, 95% Cl, 0.24-1.5) tended to be more pronounced than that associated with endometrial cancer alone or with ovarian cancer alone. CONCLUSIONS: Though limited by relatively small numbers, our results suggest that the presence of some common etiologies is a basis for the unusually high co-occurrence of endometrial and ovarian cancers.