A number of important factors determine the risk for breast cancer, and the most important of these seem to be related to estrogen and possibly prolactin. Additional research is necessary on the role of endogenous and exogenous estrogens and the effect of diet, drugs, and other factors on the levels of estrogen and prolactin. It is unlikely that with present epidemiologic evidence, breast cancer can be prevented. We cannot alter the age of menarche, and promotion of early pregnancy to protect against breast cancer is not feasible. One risk factor that is alterable is obesity, particularly in the postmenopausal woman. The risk factors associated with exogenous estrogens following the menopause require confirmation by other studies. However, because of the strong association of these drugs with increased risk of endometrial cancer, the physician should be cautious in prescribing them.