Breast cancer is associated with endogenous hormone levels, but the exact relation and underlying mechanisms remain unclear. Data from several recent epidemiologic studies suggest that a woman who experiences preeclampsia in her own pregnancy, or who was herself born to a preeclamptic pregnancy, is at reduced risk for breast cancer later in life. This paper reviews the evidence for a connection between preeclampsia and breast cancer risk, and discusses the hormonal mechanisms that might explain this association. Preeclampsia is characterized by reduced levels of estrogens and insulin-like growth factor-1, and by elevated levels of progesterone, androgens, human chorionic gonadotropin, IGF-1 binding protein, corticotropin-releasing factor, cortisol, and insulin. These factors may act both individually and synergistically to decrease breast cancer risk. The occurrence of preeclampsia during a woman's pregnancy may reflect an underlying hormonal profile that both predisposes her to preeclampsia and reduces her risk for breast cancer. In addition, the major hormonal alterations associated with preeclampsia during gestation may have lasting effects on subsequent breast cancer risk. Finally, the hormonal and nutritional environment of the womb, for which preeclampsia is a marker, may play an important role in programming lifelong risk for breast cancer in the female offspring.