Most patients with polycystic ovary syndrome (PCOS) have hyperinsulinemia; thus it has been postulated that insulin-lowering drugs, such as metformin, might be a useful long-term choice. We evaluated the effects of 6 months' administration of metformin on clinical and endocrine indices in PCOS patients. Forty-two hyperinsulinemic women with PCOS were continuously treated with metformin for 6 months. Gonadotropins, androgens (testosterone and androstenedione), insulin, sex hormone binding globulin (SHBG), lipid profile and clinical indices (menstrual length, body mass index (BMI), Ferriman-Gallwey score and waist/hip ratio (WHR)) were studied before and after metformin treatment. All women experienced a normalization of menstrual cycle length (reduction rate, 36.9%), a significant decrease in luteinizing hormone, insulin and androgen levels and an increase in SHBG plasma concentrations, with a concomitant decrease in cycle length and WHR. Significant changes were observed in the lipid profile. According to baseline androgen levels, patients were divided into two groups: 20 normoandrogenic and 17 hyperandrogenic women. The greatest decline of androgens, BMI and Ferriman-Gallwey score was observed in hyperandrogenic patients. Lowering of androgenicity was independent of BMI. Significant changes in lipid profile were observed in both groups after metformin treatment. These results suggest that metformin is effective in decreasing hyperandrogenism, mainly by reducing insulin levels. This leads to an improvement of clinical manifestations of PCOS and, in particular, of hyperandrogenism.