Previous studies on nonsteroidal antiinflammatory drugs (NSAIDs) and breast cancer have produced mixed results. Incident invasive cases of breast cancer from the Multiethnic Cohort (African Americans, Caucasians, Japanese Americans, Latinas, and Native Hawaiians from Hawaii and California) were identified from 1993 to 2002. Data on aspirin, acetaminophen, and other NSAID (ibuprofen, naproxen, indomethacin) use were based on a self-administered questionnaire at baseline (1993-1996). Multivariate Cox proportional hazards models provided estimates of hazard rate ratios and 95% confidence intervals. The authors observed no associations between breast cancer risk and duration of aspirin use for current or past users (hazard rate ratio = 1.05, 95% confidence interval: 0.88, 1.25 and hazard rate ratio = 1.04, 95% confidence interval: 0.84, 1.27 for > or =6 years of use, respectively) compared with nonusers. However, duration of current other NSAID use was protective (hazard rate ratio = 0.70, 95% confidence interval: 0.51, 0.95 for > or =6 years of use; p(trend) = 0.01) against the risk of breast cancer, while past use was not (hazard rate ratio = 0.90, 95% confidence interval: 0.62, 1.30 for > or =6 years of use). Analyses by ethnicity and hormone receptor status showed that the protective effect of current other NSAID use was limited to Caucasians and African Americans and to women with at least one positive hormone receptor. This study found duration of current other NSAID use to be protective against breast cancer risk.