Polycystic ovary syndrome (PCOS) is a heterogeneous disorder with widespread systemic manifestations affecting 5-10% of women of reproductive age. The accompanying insulin resistance and hypeinsulinemia mark this syndrome as a prediabetic state, with high incidence of impaired glucose tolerance, gestational diabetes, and overt diabetes. Other metabolic and biochemical changes, such as hypertension and dyslipidemia, increase the risk of cardiovascular disease. Fertility may also be impaired due to anovulation, impaired implantation, and higher rates of spontaneous abortions. All of these effects may also be related to hyperinsulinemia. Metformin, as insulin-sensitizing drug, is being evaluated for its potential long-term disease-modifying effect, such as prevention of diabetes. Its use may also help restore spontaneous ovulation and improve menstrual cyclicity, improve the success rate of induction of ovulation with clomiphene citrate and FSH, and decrease the high rate of ovarian hyperstimulation and early pregnancy loss. Nevertheless, these new exiting potential benefits of metformin should be evaluated in large randomized controlled studies, and clinicians must counsel women appropriately before the initiation of metformin therapy.