Polycystic ovarian syndrome (PCOS) is one of the most common endocrine diseases in women. This syndrome is characterized by hyperandrogenism, chronic anovulation, infertility and obesity. The association between PCOS-related hyperandrogenemia and insulin resistance is well documented in the literature. Insulin resistance and the resulting raised plasma levels of insulin are reported to be responsible for the high androgen concentration observed in patients with PCOS. In this prospective study, blood samples for levels of testosterone (T), dehydroepiandrosterone sulfate (DHEAS), luteinizing hormone (LH), follicle-stimulating hormone (FSH), LH/FSH, prolactin and fasting blood sugar (FBS) before starting metformin administration were obtained randomly from 40 women who were apparently obese, had PCOS and had been referred to a university hospital. Metformin was then given at a dose of 500 mg three times a day for 8 weeks, after which time the pretreatment study was repeated. Clinical symptoms of PCOS, including acne and hirsutism score and body mass index (BMI), were assessed before and after the treatment cycle. Metformin therapy resulted in a significant decrease in total testosterone levels and FBS. There was also a significant decline in BMI, length of the menstrual cycle, acne and hirsutism score. There were no significant changes in the levels of DHEAS, prolactin, FSH or LH, or in LH/FSH. The effect of metformin on subjects with elevated DHEAS levels was different to that on individuals with normal DHEAS levels. In the latter group there were only significant improvements in the length of the menstrual cycle, BMI and testosterone and DHEAS levels. It is concluded that metformin therapy in subjects with PCOS results in a decrease in fasting blood sugar and testosterone levels, and leads to a significant improvement in the clinical manifestation of hyperandrogenism. These responses also related to the level of adrenal function.