Objective: To compare the effectiveness of letrozole and clomiphene citrate (CC) in ovulatory patients with borderline male factor infertility, early stage endometriosis, and unexplained infertility. Design: Prospective quasi-random-ized trial. Setting: University infertility clinic. Patient(s): Forty-six consecutive patients with ovulatory infertility were recruited. Twenty-five patients (67 cycles) were given CC and 21 patients (52 cycles) were given letrozole. Both drugs were given orally on days 3- 7 of menses. Intervention(s): Letrozole, CC, ovulation induction, IUI, timed intercourse. Main Outcome Measure(s) : Number of follicles, endometrial thickness, and pregnancy rates. Result(s): The median serum E2 concentration on the day of hCG administration in the letrozole and CC groups were 191.5 pmol/L and 476.0 pmol/L, respectively. The median endometrial thickness on the day of hCG were 8 mm in both groups. Ovulation occurred in 81% (42/52) of the letrozole-treated and 85% (57/67) of the CC- treated patients. Pregnancy rate (PR) per cycle was 9% (5/52) in the letrozole group and 12% (8/67) in the CC group. Conclusion(s): Letrozole and CC have comparable effectiveness in ovulatory patients with borderline male factor infertility, early stage endometriosis, and unexplained infertility. Objective: To compare the effectiveness of letrozole and clomiphene citrate (CC) in ovulatory patients with borderline male factor infertility, early stage endometriosis, and unexplained infertility. Design: Prospective quasi-random-ized trial. ): Forty-six consecutive patients with ovulatory infertility were recruited. Twenty-five patients (67 cycles) were given CC and 21 patients (52 cycles) were given letrozole. Both drugs were given orally on days 3- 7 of menses. Intervention ( Results: The median serum E2 concentration on the day of hCG administration in (s): Letrozole, CC, ovulation induction, IUI, timed intercourse. Main Outcome Measure (s): Number of follicles, endometrial thickness, and pregnancy rates. the median endometrial thickness on the day of hCG were 8 mm in both groups. Ovulation occurred in 81% (42/52) of the letrozole-treated and 85% (57/67) of the CC-treated patients. Pregnancy rate (PR) per cycle was 9% (5/52) in the letrozole group and 12% (8/67) in the CC group. Conclusion (s): Letrozole and CC have comparable effects in ovulatory patients with borderline male factor infertility, early stage endometriosis, and unexplained infertility.