The polycystic appearance of the ovary is the distinguishing characteristic of polycystic ovary syndrome (PCOS) but there is a wide range of other clinical and biochemical features, e.g. elevated serum concentrations of androgens, insulin, LH and decreased insulin sensitivity. The high prevalence of affected individuals and the wide range of related phenotypes can be explained by the interaction of a small number of key genes with environmental factors. Heritability of PCOS has been inferred from studies of the syndrome in various populations (ethnic groups, twins and PCOS families). The data suggest that the condition is passed down through either sex, according to an autosomal dominant model of genetic transmission. To date, specific gene mutations affecting androgen synthesis, insulin secretion and insulin activity explain most of the endocrine and metabolic symptoms, while environmental risk factors (either during prenatal or post-natal life), seem to convert an occult PCOS into a clinically manifest syndrome.