Epidemiology and the causes of breast cancer.

This article describes the characteristics of 3 dominant features of breast cancer epidemiology. These characteristics include the association of disease risk with childbearing, its relationship to ovarian activity and its international variation (particularly as the latter differs in the years before and after menopause). Equivocal tests of one hypothesis that reconciled some of these features through variations in levels of the fractions of estrogen are described. Other hypotheses with a similar objective are needed. The 3 known causes of human breast cancer, ionizing radiation, exogenous ovarian hormones and beverage alcohol, offer some preventive possibilities but do little to explain the epidemiologic features of the majority of cases of the disease that occur in their absence.