Polycystic ovary syndrome (PCOS) is the commonest endocrine disorder in women of reproductive age. It is the major cause of anovulatory infertility and of hirsutism but its aetiology remains uncertain. The most consistent biochemical abnormality is hypersecretion of androgens. In this review, evidence is presented to support the view that a genetically determined abnormality of ovarian androgen secretion is the primary cause of the syndrome. Molecular genetic studies have demonstrated that CYP11a, the gene encoding P450 cholesterol side chain cleavage, is a major susceptibility locus for development of hyperandrogenism in PCOS.