Many clinical observations have suggested that there is a strong relationship between hyperinsulinemia and hyperandrogenism. HAIR-AN syndrome is defined as a constellation of hyperandrogenism (HA), insulin resistance (IR), and acanthosis nigricans (AN). Two major mechanisms could account for this syndrome: (i) hyperinsulinemia induced by insulin resistance causes hyperandrogenism, and (ii) hyperandrogenism causes insulin resistance and hyperinsulinemia. Acanthosis nigricans is considered to be an epiphenomenon caused by hyperinsulinemia. The causes of HAIR-AN syndrome include syndromes of extreme insulin resistance due to mutations in the insulin receptor gene, lipoatrophic diabetes, obesity, some endocrinopathies, and genetic and acquired ovarian hyperandrogenism.