Two long and broad streams of medical literature, from the 1950's to date, have established the existence of two unrelated abnormalities of androgen production in women with breast cancer. One is the genetically determined presence of subnormal production of adrenal androgens (i.e. DHEA and DHEAS) in women with premenopausal breast cancer and their sisters, who are at increased risk for breast cancer. The other is excessive production of testosterone, of ovarian origin, in subsets of women with either premenopausal or postmenopausal breast cancer and women with atypical breast-duct hyperplasia, who are at increased risk for breast cancer; along with the hypertestosteronism, there is frequently chronic anovulation in the premenopausal patients. The combination of ovarian hypertestosteronism and chronic anovulation is characteristic of the polycystic ovary syndrome and is also frequently seen in women with abdominal ("android") obesity; both PCOS and abdominal obesity are known to be characterized by high risk for postmenopausal cancer. The elevated testosterone levels and the increased levels of insulin, IGF-I, and IGF-II that are seen in PCOS and abdominal obesity could favor the development of breast cancer in several ways, all of which have been demonstrated experimentally: binding of testosterone to cancer cells bearing testosterone receptors, with direct stimulation; intratissular aromatization of testosterone to estradiol, with stimulation of estrogen-sensitive cells; stimulation of the production of epithelial growth factor (EGF) by testosterone, with direct mitogenic effect of EGF on cancer cells; stimulation of aromatase by insulin and IGF-I; direct mitogenic stimulation of cancer cells by insulin, IGF-I, and IGF-II; and stimulation by IGF-I and IGF-II of the intratissular reduction of estrone to estradiol. Since PCOS is probably largely genetically determined, and abdominal obesity may also be, the hypertestosteronism of these conditions may represent a second genetically determined hormonal risk factor for breast cancer.