Hormone replacement therapy increases the risk of breast cancer.

Established risk factors for breast cancer predominantly act through hormonal pathways. The major exceptions are (1) family history, which accounts for approximately 6 percent of the cases of breast cancer in the United States, (2) radiation, particularly in adolescence, which accounts for a small fraction of all cases of breast cancer, and (3) personal history of benign breast disease, which may include precursor lesions in the pathway to breast cancer. Age at menarche, parity, and age at first birth, as well as age at menopause are related to risk of breast cancer. In addition, among postmenopausal women, relative weight and use of postmenopausal hormones increase risk of breast cancer. This paper reviews the evidence that these factors may act through hormonal pathways or cell division in response to cyclic hormonal levels.