Background: Several epidemiologic studies have shown inverse associations between use of nonsteroidal anti-inflammatory drugs (NSAID) and incident ovarian cancer, but the results are inconsistent. There have been only a few studies examining possible links between NSAIDs and endometrial cancer risk. We investigated associations between use of NSAIDs and incident ovarian and endometrial cancers in a prospective cohort of about 20,000 women with ages from 58 to 76 years in 1992. Participants were asked how often they used aspirin and nonaspirin NSAIDs. Over 15 years, 311 endometrial and 167 ovarian incident malignancies were identified. Multivariate-adjusted hazard ratios were estimated using Cox proportional regression. Results: Compared with women who reported no use of aspirin, the relative risks for ovarian cancer for those who used aspirin <2, 2 to 5 times, and ≥6 times per week were 0.83, 0.77, and 0.61, respectively (P trend = 0.04). We did not observe any association between nonaspirin NSAIDs use and ovarian cancer risk. Neither did we find associations between aspirin or nonaspirin NSAIDs use and risk for endometrial cancer. Conclusions: Our results suggest a possible inverse association between frequency of aspirin use and risk for ovarian cancer.