Purpose: To compare the effects of letrozole, clomiphene citrate and metformin plus letrozole on ovulation induction in infertile women with polycystic ovarian syndrome. Study design: Prospective, randomized clinical study Material and method: Thlis study was conducted on 15 patients with polycystic ovarian syndrome, who admitted to infertility clinic of Erciyes University, Faculty of Medicine between March 2004 and November 2005, Patients were randomized into three treatment groups, In group 1, continuous metformin was used at the dose of 850mg/tid/day for six months; afterwards, daily 2.5mg letmzoie between 3 and 7 days of the menstrual cycle was added to the metformin therapy, Group 2 patients received only daily 2,5mg letrozole between days 3 and 7 of the menstrual cycle and group 3 patients received daily 100mg clomiphene citrate only between clays 3 and 7 of the menstrual cycle. The patients were treated until either a pregnancy occurred, or six cycles were reached without pregnancy. Results: There were five patients group 1, five women in group 2 and five women group 3. Endometrial thickness of patients treated with clomiphene citrate (6.8 +/- 0.5) was only sigrilficantly thinner when compared to those with the letrozole only (10,4 1,4) and letrozole plus metformin groups (9.6 +/- 1.3). Serum estradiol on the day of hCG administration was significantly higher in the clomiphene citrate group mum Conclusion: Letorzole may have a beneficial effect on ovulation induction in patients with polycystic ovarian syndrome.