Polycystic ovary syndrome (PCOS) is a complex and heterogeneous disorder characterized by hyperandrogenemia, hyperinsulinemia, insulin resistance, and chronic anovulation. It is the most common endocrine disorder in women of reproductive age with an enigmatic pathophysiologic and molecular basis. The high prevalence of affected individuals and the wide range of phenotypic expression can be explained by the interaction of a number of key genes with environmental factors. Heritability of PCOS has been inferred from studies of the syndrome in various population groups (ethnic groups, twins, and PCOS families). Although evidence of familial segregation and clustering of the disease in first-degree relatives of women diagnosed with PCOS has been presented, no particular pattern of inheritance has emerged. Some of the problems in genetic studies have been the lack of uniform criteria for diagnosis, heterogeneity of phenotypic features, and the fact that the disorder is only expressed clinically in women during their reproductive years. Even within affected families and between sisters with polycystic ovaries, there is heterogeneity in presentation. However, regardless of diagnostic criteria used to identify the propositus and to determine affected status in the kindred, the genetic studies available suggest a strong familial component. Currently, PCOS is considered a polygenic trait that might result from the interaction of susceptible and protective genomic variants and environmental factors, during either prenatal or postnatal life.