Polycystic ovary syndrome (PCOS) can be more difficult to diagnosis in adolescents than adults. Although not all adolescent females with obesity have PCOS, and not all females with PCOS have obesity, insulin resistance related to co-existent obesity is the most common non-steroidogenic cause. PCOS runs in families -- about half of sisters are hyperandrogenic and half of those have PCOS. Although physiologic adolescent anovulation is a well-known phenomenon, normal adolescent menstrual cyclicity differs only slightly from normal adult cyclicity; cycles shorter than 19 days or longer than 90 days are abnormal even in adolescents. Even though most cycles in the first post-menarchal year are anovulatory, bleeding usually occurs at 21-45 day intervals. Insulin resistance and obesity are commonly present but not necessarily present, so they are not diagnostic criteria. If insulin resistance and obesity are present, the insulin resistance is often out of proportion to that conferred by the obesity. Metabolic syndrome is present in 25 percent of adolescents with PCOS. The initial workup includes total testosterone, free testosterone, SHBG, DHEAS, and an early morning 17-OH progesterone level. Other labs are individualized. The main considerations in treatment are menstrual irregularity, cutaneous manifestations, and the co-morbidities of metabolic syndrome. Combined oral contraceptive pills are the first line of treatment. Progestin monotherapy may be used alternatively, although it is not as effective at controlling bleeding and suppressing androgens. Weight loss and increased exercise is key. Insulin-lowering treatments may be used in conjunction with lifestyle modification. Well-controlled studies have shown no advantage to metformin over lifestyle modification, but metformin may have an additive benefit if it's used in combination with lifestyle modification. The clear indication for metformin is abnormal glucose tolerance. An early workup for PCOS is encouraged so that a diagnosis can be made prior to initiating treatment. If treatment has been initiated within the first two post-menarchal years, a three-month trial off of OCPs can be attempted when the patient is gynecologically mature to determine the persistence of hyperandrogenic anovulation. Contraceptive counseling should occur at that time because infertility is relative, not absolute.