OBJECTIVES: Polycystic Ovary Syndrome (PCOS) is one of the most common endocrine disorders affecting five to ten percent of young women. Aside from its cutaneous symptoms and reproductive morbidity, PCOS associates with metabolic syndrome and an increased risk of cardiovascular events. In recent years the use of Metformin, an insulin sensitizer, in PCOS has aroused a great interest. Metformin administration restores ovulatory menstrual cycles and seems to improve infertility. However, the extent to which this agent improves the metabolic and cardiovascular risk factors associated with PCOS remains uncertain. Our aim was to assess the effectiveness of Metformin in improving clinical and metabolic features of PCOS. METHODS: Systematic Review (up to February 2008) and meta-analysis of randomized controlled trials that compared Metformin versus placebo or other standard therapies for PCOS such as oral contraceptive pills (OCPs) and measured metabolic and cardiovascular parameters. Studies were critically appraised. Pooled estimates of effect were based on a random-effects model. Odds Ratio and Weighted Mean Difference, with 95% confidence interval, used were appropriate. Consistency across studies was evaluated by means of the I-square statistic. Potential reasons for heterogeneity were explored.RESULTS: Of 120 candidate studies, 19 trials were eligible. Methodological quality of these trials was low. Meta-analyses showed a small decrease in BMI and systolic blood pressure in women treated with Metformin compared to placebo. The other parameters including waist circumference, waist-hip ratio, diastolic blood pressure, fasting blood glucose, insulin levels and lipid profile did not differ between the groups. As against OCPs, Metformin significantly reduced the BMI but no other significant differences were observed. CONCLUSIONS: Scant and inconsistent evidence suggest that Metformin provide limited or no important benefit for metabolic derangements and cardiovascular risk factors in women with PCOS. Further research is needed to solve this important health issue.