Chronic inflammation may play an etiologic role in endometrial cancer. Nonsteroidal anti-inflammatory drugs (NSAIDs) reduce inflammatory activity by inhibiting the proinflammatory cyclooxygenase enzymes and, therefore, may decrease cancer risk. However, few studies have examined the association between NSAID use and endometrial cancer. We conducted a prospective study among 72,524 women in the NIH-AARP Diet and Health Study. Women completed a questionnaire in 1996-1997 on lifestyle and health-related factors, including type and frequency of NSAID use within the past year, and were followed through 2003 by linkages to cancer registries and vital status databases. During 488,261 person-years of follow-up, there were 732 incident endometrial cancers. NSAID use, compared with nonuse of NSAIDs, was not significantly associated with endometrial cancer risk [relative risk (RR), 0.90; 95% confidence interval (95% CI), 0.74-1.09]. Null associations were also observed by type of NSAID use [aspirin only: RR, 0.88; 95% CI, 0.70-1.11; nonaspirin NSAID (NA-NSAID) only: RR, 1.01; 95% CI, 0.79-1.29; both aspirin and NA-NSAIDs: RR, 0.85; 95% CI, 0.68-1.06]. Generally, results were not statistically significant by frequency of use for aspirin or NA-NSAIDs. Results did not change when women with a history of heart disease, hypertension, or diabetes were excluded to minimize the potential for confounding by indication. Overall, our data do not support an association between aspirin or NA-NSAID use and endometrial cancer risk.