41 women with polycystic ovary syndrome (PCOS) and 15 healthy women of reproductive age were examined to evaluate ovarian aromatase activity. Aromatase activity was determined by the decrease of estradiol level after peroral intake of aromatase inhibitor letrosol. To examine aromatase activity of antral follicle (∆E2) was divided on the blood level of antimullerian hormone (AMH), which is corresponded to the number of antral follicles. Significant variations of aromatase activity of antral follicles in patients with PCOS were determined: in 34.1 % of women it was within physiological ranges, in 48.8 % of women it was decreased and in 17.1 % of women it was increased. Aromatase activity of antral follicles in patients with PCOS correlated with blood levels of estradiol (r = 0.67), estron (r = 0.27), free testosterone(r = 0.43), androstendion (r = 0.34) and body mass index (r = 0.30). Aromatase activity had reverse correlation with number of antral follicles. Athors suggest that the sensitivity of the ovaries to gonadotropinic stimulation is decreased in patients with PCOS and low aromatase activity.