The objective of this study was to examine the association between nonsteroidal anti-inflammatory drug (NSAID) use and the development of breast cancer, and to assess whether this association differed by estrogen receptor (ER) subtype. Data were analyzed from 15,651 women participating in CLUE II, a cohort study initiated in 1989 in Washington County, MD. Medication data were collected at baseline in 1989 and in 1996. Incident cases of invasive breast cancer occurring from baseline to March 27, 2006 were identified through linkage of cohort participants with the Washington County Cancer Registry and the Maryland State Cancer Registry. Cox proportional hazards modeling was used to calculate the risk ratios (RR) and 95% confidence intervals (95% CI) for breast cancer associated with medication use. Among women in the CLUE II cohort, 418 invasive breast cancer cases were identified during the follow-up period. The results showed that self-reported use of NSAIDs in both 1989 and in 1996 was associated with a 50% reduction in the risk of developing invasive breast cancer compared with no NSAID use in either 1989 or 1996 (RR = 0.50; 95% CI 0.28, 0.91). The protective association between NSAID use and the risk of developing breast cancer was consistent among ER-positive and ER-negative breast cancers, although only the RR for ER-positive breast cancer was statistically significant. Overall, findings from this study indicate that NSAID use is associated with a decrease in breast cancer risk and that the reduction in risk is similar for ER-positive and ER-negative tumors.