Polycystic ovarian syndrom (PCOS) affects 5% to 10% of women of reproductive age, and is the most common reason of anovulation in infertile women. It is multi-symptom disease. A drug which is used to induce ovulation in women with PCOS is clomiphene citrate. After clomiphene treatment, ovulation is achieved in 56%-73% of women. The rest of them has different level of opportunity to therapy. Clomiphene does not have influence on elementary factor in PCOS which seems to be opportunity to insulin. The drug which can have positive effect is metformin, derivative from biguanid. Its influence on percent of ovulation, pregnancy and live births was evaluated based on randomized trials. Many trials have reserched this so far, but their results are divergent. In this article, we are aiming to systemize results of randomized trials concerning the role of metformin in PCOS therapy.