The polycystic ovary syndrome is the most common endocrinopathy in reproductive age women. The pathophysiology of this syndrome remains unclear, but there are evidences that a decreased in insulin sensitivity may be related in this syndrome. There are studies that showed the action of factors on the sensitivity of the insulin receptor, such as melatonin. It is a hormone produced by the pineal gland, whose role is still uncertain in human reproduction, although many studies have found that it may relate sex hormone effect. Many patients with polycystic ovary syndrome (PCOS) may have hyperinsulinemia, and that pinealectomized rats determined hypoglycemia and hyperinsulinemia, with reduced release of insulin in response to glucose. Therefore, it is supposed that blood levels of melatonin might correlate with the blood insulin concentration in patients with the syndrome. Moreover, the insulin sensitizing agents have been used in the treatment of patients with insulin resistance and PCOS, based on its main pathophysiological substrate which is the hyperinsulinemia. In fact, the metformin is the most used in the literature. However, there are few studies on the use of metformin in adolescents. The evidences of this drugs in this group of patients have showed good therapeutic response with few side effects. The objective of the study is to evaluate the effects of metformin on insulin resistance and levels of melatonin in adolescents with PCOS. It will be performed a prospective, randomized, double-blind and placebo-controlled study with 90 adolescents with PCOS. It will be evaluated clinical and laboratory parameters (TSH, free T4, prolactin, FSH, LH, estradiol, total testosterone, androstenedione, DHEA-s, 17-OH progesterone, SHBG, free androgen index, index of HOMA-IR and QUICKI from fasting glucose and insulin, total cholesterol and fractions, triglycerides, creatinine, AST and ALT and creatinkinase, fibrinogen and PAI-1, and melatonin for 6 months of treatment.