Polycystic ovary syndrome (PCOS) is a syndrome of variable combinations of menstrual irregularity, hirsutism or acne, and obesity. It can be diagnosed in adolescence and has early childhood antecedents. PCOS is the single most common endocrine cause of an ovulatory infertility and a major risk factor for the metabolic syndrome and, in turn, development of type 2 diabetes mellitus in women. Thus, it appears that PCOS increases a woman's risk of developing cardiovascular disease. Therefore, identifying girls at risk for PCOS and implementing treatment early in the development of PCOS may be an effective means of preventing some of the long-term complications associated with this syndrome. This article reviews the definition, clinical features, diagnosis, and treatment of PCOS.