Prevention has long been the holy grail of breast cancer research. The significant reduction in breast cancer risk afforded by a full-term pregnancy early in life suggests the great potential of preventive strategies. In contrast to the risks associated with prolonged exposures, exogenous estrogen and progesterone for short durations can mimic the protective effects of pregnancy in carcinogen-induced mammary tumor models. Rajkumar and coworkers have now demonstrated that these hormones protect mice from mammary tumors initiated by a spectrum of oncogenic alterations that are common in breast cancers. Although differences between rodent models and humans remain, the results reveal that exogenous estrogen and progesterone potently inhibit tumorigenesis through multiple pathways and establish a foundation for strategies to prevent breast cancer.