WHAT IS KNOWN AND OBJECTIVES:Letrozole is widely known for its use as an ovulation inductor. This study aims to investigate the effects of letrozole and clomiphene citrate in females with polycystic ovarian syndrome. METHODS:This is a randomized non-blinded controlled trial study that included 80 infertile females with polycystic ovarian syndrome receiving a standard dose of either clomiphene citrate or letrozole on day 2 of the cycle. An ultrasound was done to examine growth of the follicle, endometrial thickness on days 12-13, and a Doppler study to measure resistance index (RI), pulsatility index and ratio of systolic/diastolic velocity. RESULTS AND DISCUSSION:The mean levels of dominant follicle and oestradiol were significantly higher in the clomiphene citrate group than in the letrozole group. The letrozole group had a significantly greater endometrial thickness than the clomiphene citrate group. The resistance index and pulsatility index were lower in the letrozole group and in pregnant women than in the clomiphene citrate group and the non-pregnant group. WHAT IS NEW AND CONCLUSION:The use of letrozole for ovulation induction in polycystic ovarian syndrome patients has a better effect on endometrial receptivity markers when compared to clomiphene citrate.