CONTEXT: due to the central role of metabolic abnormalities in the pathophysiology of PCOS, insulin sensitising agents have been proposed as a feasible treatment option. OBJECTIVE: to investigate which is the more effective between metformin and myoinositol on hormonal, clinical and metabolic parameters in obese patients with polycystic ovary syndrome (PCOS). STUDY DESIGN: crossover randomized-controlled study. PATIENTS: Thirty-four PCOS obese women (age: 25.62 ± 4.7 years; BMI: 32.55 ± 5.67 kg/m2 ) were randomized to receive metformin (850 mg twice a day) or myoinositol (1000 mg twice a day) for six months. After a three month washout, the same subjects received the other compound for the following six months. MEASUREMENTS: Ultrasonographic pelvic examinations, hirsutism score, anthropometric and menstrual pattern evaluation, hormonal profile assays, oral glucose tolerance test (OGTT) and lipid profile at baseline and after 6 months of treatment were performed. RESULTS: Both metformin and myoinositol significantly reduced the insulin response to OGTT and improved insulin sensitivity. Metformin significantly decreased body weight, and improved menstrual pattern and Ferriman-Gallwey score. Metformin treatment was also associated with a significant decrease in LH and estradiol levels, androgens and AMH levels. None of these clinical and hormonal changes were observed during myoinositol administration. CONCLUSIONS: Both treatments improved the glyco-insulinaemic features of obese PCOS patients,but only metformin seems to exert a beneficial effect on the endocrine and clinical features of the syndrome. This article is protected by copyright. All rights reserved. This article is protected by copyright. All rights reserved.