In a recent prospective study evaluating the efficacy of 1700 mg/day metformin as first-line approach for infertile anovulatory patients with PCOS, we identified predictors for metformin efficacy. Our analysis demonstrated that body mass index (BMI) and insulin resistance were the strongest predictors for both ovulation and pregnancy. In particular, adjusting the data for insulin resistance, a trend in reduced effectiveness was observed with increasing BMI. On the other hand, adjusting the data for BMI, a trend in improved efficacy was detected for higher insulin resistance degrees.