The polycystic ovary syndrome (PCOS) is characterized by hyperandrogenism and insulin resistance, but the connection between these two features has been unclear. Androgen synthesis is regulated in part by the ratio of the 17alpha-hydroxylase and 17,20 lyase activities of P450c17. Three separate lines of evidence show that the ratio of lyase to hydroxylase activity is regulated by electron flow from P450 oxidoreductase. Lyase activity and androgen synthesis are particularly dependent on the serine phosphorylation of P450c17. Serine phosphorylation of the insulin receptor beta chain causes insulin resistance, and some PCOS women have hyperphosphorylated receptors. We hypothesize that an overactive serine/threonine kinase hyperphosphorylates both the insulin receptor and P450c17 in PCOS, accounting for the characteristic insulin resistance and hyperandrogenism of this disease.