Cancers of the breast, endometrium, and ovary account for 41% of incident cancers among women. Many risk factors for breast cancer have been identified, but most are associated with only modest elevations in risk. Also, of all the risk factors identified, few are likely to be affected by intervention programs. The pathogenesis of breast cancer is not well understood, but estrogen and possibly estrogen plus progesterone are likely to be etiologically involved. For endometrial cancer, a major etiologic pathway is exposure to estrogen without cyclic exposure to progesterone. Most of the established risk factors for endometrial cancer appear to affect risk at least in part through this pathway. Only a few risk factors for ovarian cancer have been identified. The two most commonly suggested etiologic mechanisms for ovarian cancer are (a) that suppression of ovulation reduces risk and (b) that suppression of pituitary gonadotropins reduces risk. Each hypothesis is consistent with some, but not all, of the data. Prospects for the primary prevention of these cancers are discouraging at present because few of the risk factors identified to date are readily subject to modification, especially for breast cancer, and the underlying etiologies of these cancers are not well understood, particularly for breast and ovarian cancers.