To investigate the effects of ovulation induction using letrozole and clomiphene citrate in combination with rFSH on outcome of IUI cycles in those suffering from infertility. This was a randomized clinical trial being performed in Shiraz Ghadir hospital and our private infertility clinic, during a 24-months period. 144 women with unexplained infertility were randomly divided into two groups to receive ovarian superovulation. Group A (n=72) received 5-mg per day letrozole (Razak Drug Laboratory, Tehran, Iran) for 5 days from the 5th to the 9th day of cycle while Group B (n=72) received 100-mg per day clomiphene citrate (Razak Drug Laboratory, Tehran, Iran) for 5 days in the same period. All patients received highly purified recombinant FSH (Gonal-f, Serono, Hellas, Puregon, Greece) was injected subcutaneously in a dosage of 150 unit/day from the 8th day of the cycle until the day of hCG administration. Transvaginal sonography was performed on the day 11th of the cycle and according to the size and number of stimulated follicles, rFSH was continued till at least two dominant follicles with size of >18mm were seen. Then 10000 units of human chorionic gonadotropin (hCG) (Choriomon, IBSA, Switzerland) was injected intramuscularly, if serum E2 level was below 2000pg/ml. The endometrial thickness (ET) was measured the same day at its greatest diameter perpendicular to the midsagittal plane at the fundal region of the uterus. A single IUI was performed 36-40 hours after hCG administration. Main outcomes included pregnancy rate, pregnancy rate per cycle, abortion and endometrial thickneness.