Major aspects of the epidemiology of breast cancer.

Current understanding of the major aspects of breast cancer epidemiology is reviewed. These include exogenous factors such as viruses, and the use of oral contraceptives and of estrogens. Endogenous factors are also described, especially those relating to reproductive experience. Current endocrine hypotheses relating to breast cancer etiology are described, and the estrogen balance hypothesis is presented in detail. A new area of epidemiologic research is now developing rapidly. This relates to the value of the parenchymal pattern as a marker of risk in young women. The role of this factor per se and in relation to other risk factors is described. Current approaches to breast cancer control are inadequate but will be discussed.