The evidence that supports the hypothesis that hyperinsulinemia causes ovarian hyperandrogenism was reviewed. The most dramatic clinical expression of the association between hyperinsulinemia and hyperandrogenism is the HAIR-AN syndrome. In the HAIR-AN syndrome, severe insulin resistance results in a compensatory hyperinsulinemia that stimulates ovarian androgen production. Acanthosis nigricans, a dermatologic manifestation of severe insulin resistance, is an epiphenomenon of the disease. Hyperandrogenic (HA) women can be divided into two major groups: insulin-resistant (HA-IR) and non-insulin-resistant (HA-nIR). HA-nIR women have markedly elevated LH, often have minimally elevated serum prolactin, and have polycystic ovaries. HA-IR women have minimally elevated LH, markedly elevated plasma insulin, and stromal hyperthecosis. Women with HA-nIR probably have a primary hypothalamic-pituitary abnormality as the cause of their hyperandrogenism. Women with HA-IR probably have a primary metabolic abnormality as the cause of their hyperandrogenism. The relationship between hyperinsulinemia and hyperandrogenism is one example of the complex interrelationships that exist between central metabolism and reproduction.