The evidence that various endogenous and exogenous sex hormones play a role in the etiology of breast cancer is reviewed. It is hypothesized that prolonged exposure to normal levels of ovarian estrogens and cyclic progesterone resulting from early menarche and late menopause, and primarily willful nulliparity and late childbearing, act at an early stage in the development of breast cancer by promoting excessive proliferation of normal epithelial stem cells. Excess endogenous or exogenous estrogens can enhance risk by stimulating proliferation of epithelial cells that have undergone partial malignant transformation. The breast, however, is much less responsive to the tumor-promoting effects of estrogens than the endometrium, and estrogens probably play a less important role in the later stages of mammary than endometrial carcinogenesis.