Aspirin and other NSAIDs, such as ibuprofen, have been found in preclinical in vitro, in vivo, and epidemiologic studies to have potential utility in breast and colon cancer risk reduction by several mechanisms.1 These mechanisms include inhibition of COX-2, cell proliferation, angiogenesis, and metastasis and induction of apoptosis.2 However, for breast cancer chemoprevention (cancer risk reduction), COX-2 inhibition may specifically decrease prostaglandin induction of aromatase, the enzyme responsible for converting androgens to estrogen in breast tissue.3 Aspirin and other NSAIDs may, therefore, reduce risk of ER+ breast cancer, as suggested by epidemiologic observational data to date, but not ER− breast cancer.4