['To determine the clinical, hormonal, and biochemical effects of 4-6 months of metformin therapy in obese patients with polycystic ovary syndrome (PCOS).', 'Prospective study.', 'The Gynecological Endocrine Unit of University Central Hospital, Oulu, Finland.', 'Twenty obese patients with PCOS.', 'Patients were treated with 0.5 g of metformin three times daily for 4-6 months.', 'Clinical symptoms, menstrual pattern, and hirsutism, as well as serum concentrations of sex steroids, sex hormone-binding globulin (SHBG), gonadotropins, and lipids were assessed during the treatment.', 'Eleven women (68.8% of the women with menstrual disturbances) experienced more regular cycles during therapy. No changes in hirsutism, body mass index, or blood pressure occurred. The mean testosterone level was decreased significantly after 2 months of treatment but returned to the starting level by 4-6 months. Free testosterone levels decreased significantly during the treatment. There was no significant change in the levels of other sex steroids or lipids measured at 4-6 months of treatment.'] Does metformin therapy improve the menstrual pattern with minimal endocrine and metabolic effects in women with polycystic ovary syndrome?