We evaluated the fertility promoting effect of metformin in infertile patients with polycystic ovary syndrome. Twenty-nine infertile patients with polycystic ovary syndrome (PCOS) are included in our prospectively designed study and 15 normal menstruating women served as controls for reproductive hormones and ovarian volumes. All PCOS patients received a total of 78 cycles of clomiphene citrate (CC) in the beginning, then patients who could not get pregnant were switched to metformin plus clomiphene citrate. PCOS patients served as their own controls for the ovulation and pregnancy rates. At the end of the CC cycles 4.2% of patients got pregnant and 65.2% of the remaining group got pregnant with metformin plus CC cycles (p=0.0001). We have not observed any serious side effects of metformin. The high pregnancy rate of our study population is consistent with the hypothesis that insulin resistance plays an important role in the pathogenesis of anovulation in patients with PCOS.