Polycystic ovary syndrome (PCOS) affects 5-10% of women at reproductive age, and is the most common reason for hyperandrogenism and chronic anovulation. Some patients with PCOS are insulin-resistant. Also, lowered sex hormone-binding globulin (SHBG) concentration is usually observed. As a consequence, the amount of free and biologically active androgens increases. This implies that, by improving insulin and carbohydrate metabolism, metformin administration in PCOS patients could indirectly contribute to increase SHBG concentration. The aim of the present study was to assess the effects of metformin treatment in PCOS patients both with and without insulin resistance. Thirty-six patients completed treatment. Body mass index (BMI) was considerably reduced after therapy. Statistically significant decreases were noted in luteinizing hormone (LH) and fasting insulin concentrations and the free androgen index (FAI), and significant increases in follicle-stimulating hormone (FSH)/LH ratio and SHBG concentration. In the insulin-resistant group, BMI and fasting insulin concentrations were reduced considerably after treatment, and SHBG increased slightly. In the group of patients without insulin resistance, BMI, LH and FAI showed significant reductions, and FSH/LH and SHBG considerable increases. Considering the favorable effects of metformin treatment in PCOS patients both with insulin resistance and without it, it is purposeful to use this drug in both groups of women.