Chronic inflammation is associated with increased risk of multiple cancers, including breast cancer. Adipose tissues produce proinflammatory cytokines, and obesity is a risk factor for postmenopausal breast cancer. We evaluated the association of regular use of nonsteroidal anti-inflammatory drugs (NSAIDs) with breast cancer risk, overall and by body mass index (BMI) and tumor subtypes defined by estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 status. We conducted a population-based, case-control study involving 5,078 women aged 25-75 years who were recruited primarily from the Nashville metropolitan area of Tennessee. Multivariate unconditional logistic regression models were used to estimate odds ratios and 95 % confidence intervals for breast cancer risk after adjusting for multiple potential confounding factors. Regular use of any NSAID was associated with significantly reduced breast cancer risk (OR 0.78; 95 % CI 0.69-0.89). This association was observed for regular use of baby aspirin only (OR 0.82, 95 % CI 0.69-0.99), other NSAIDs only (OR 0.81, 95 % CI 0.69-0.95), and both baby aspirin and other NSAIDs (OR 0.52, 95 % CI 0.40-0.69). These significant inverse associations were found among overweight women (BMI ≥25 kg/m(2)) overall and by subtypes of breast cancer, but not among women with BMI <25 kg/m(2) (P for interaction = 0.023). Regular use of NSAIDs was inversely associated with breast cancer risk, particularly among overweight women. Overweight women may benefit more from the protective effects of NSAID use than normal-weight women.