Evidence suggests that hyperinsulinemic insulin resistance may increase serum levels of ovarian androgens and reduce sex hormone-binding globulin (SHBG) levels in humans. The present study was conducted to assess the effect of administration of the biguanide metformin, a drug commonly used in the treatment of diabetes mellitus, on androgen and insulin levels in 24 hirsute patients. The patients selected for the study were obese, with a body mass index higher than 25 kg/m2 and high fasting insulin (> 90 pmol/L) and low SHBG levels (< 30 nmol/L). All patients were given a low calorie diet (1500 Cal/day) and randomized for either metformin administration at a dose of 850 mg or a placebo, twice daily for 4 months, in a double blind study. In the placebo group, diet resulted in a significant decrease in body mass index (30.8 +/- 1.0 vs. 32.7 +/- 1.5 kg/m2; P < 0.0001), fasting insulin (127 +/- 11 vs. 156 +/- 14 pmol/L; P < 0.01), non-SHBG-bound testosterone (0.19 +/- 0.02 vs. 0.28 +/- 0.03 nmol/L; P < 0.02), androstenedione (5.8 +/- 0.5 vs. 9.0 +/- 1.1 nmol/L; P < 0.03), and 3 alpha-diolglucuronide (8.6 +/- 1.1 vs. 11.7 +/- 1.9; P < 0.005) plasma concentrations and a significant increase in the glucose/insulin ratio (0.047 +/- 0.005 vs. 0.035 +/- 0.003; P < 0.001) and plasma concentrations of SHBG (26.0 +/- 3.3 vs. 19.1 +/- 1.9 nmol/L; P < 0.001) and dehydroepiandrosterone sulfate (8.7 +/- 1.5 vs. 8.4 +/- 1.3; P < 0.05). Beneficial effects of diet were not significantly different in the patients who were given metformin instead of placebo. These results confirm that weight loss induced by a low calorie diet is effective in improving hyperinsulinemia and hyperandrogenism in obese and hirsute women. With our study design, metformin administration had no additional benefit over the effect of diet.