Polycystic ovary syndrome (PCOS) is now recognized as an important metabolic and reproductive disorder. It is associated with substantial defects in insulin action and secretion that confer a markedly increased risk for type 2 diabetes mellitus. Insulin resistance modifies reproductive function both by the direct actions of insulin on steroidogenesis and by disruption of insulin signaling pathways in the central nervous system. These insights have led to a new therapy for PCOS with insulin-sensitizing agents. Hyperandrogenemia and insulin resistance cluster in PCOS families, consistent with a genetic susceptibility to these abnormalities. There is evidence for both linkage and association of the hyperandrogenemia phenotype with an allele of a marker locus on chromosome 19, in the region of the gene encoding the insulin receptor.