Objective: The aim of this study was to evaluate the effects of either a 2.5-mg or a 5-mg daily dose of letrozole in women undergoing superovulation and intrauterine insemination (IUI). Design: Prospective randomized trial. Setting: Academic teaching hospital. Patient(s): Women < 40 years old, with patent fallopian tubes and infertility of > 1 year in duration. Intervention(s): Patients were randomized into either a 2.5-mg dose of letrozole (34 patients) or a 5-mg dose of letrozole (38 patients) daily for 5 days. When the leading follicle reached 18 mm in diameter, ovulation was triggered by an injection of hCG and IUI was performed 24 and 48 hours later. Main Outcome Measure(s): The number of follicles, endometrial thickness, and pregnancy rate. Result(s): Compared with those treated with 2.5 mg of letrozole, the total number of follicles was significantly higher in patients receiving 5 mg of letrozole. No difference in the endometrial thickness was found between the two groups. The pregnancy rate per cycle in patients receiving 5 mg of letrozole was statistically higher than in patients receiving 2.5 mg of letrozole (26.3%vs 5.9%, P < .05). No multiple pregnancies occurred. Conclusion(s): Compared with the daily dose of 2.5 mg, 5 mg of letrozole is associated with more follicles and a higher pregnancy rate. It appears that 5 mg daily for 5 days is a preferable letrozole dose for superovulation. Objective: The aim of this study was to evaluate the effects of either a 2.5-mg or a 5-mg daily dose of letrozole in women undergoing superovulation and intrauterine insemination (IUI). Design: Prospective randomized trial. Setting: Academic teaching hospital. Interventions (s): Patients were randomized into either a 2.5-mg dose of letrozole (34 patients) or a 5-mg When the leading follicle reached 18 mm in diameter, ovulation was triggered by an injection of hCG and IUI was performed 24 and 48 hours later. Main Outcome Measure (s): The number of follicles, endometrial thickness, and pregnancy rate. Results (s): Compared with those treated with 2.5 mg of letrozole, the total number of follicles was significantly higher in patients receiving 5 mg of letrozole. No difference in the endometrial thickness was found between the two groups. The p regnancy rate per cycle in patients receiving 5 mg of letrozole was marked higher than in patients receiving 2.5 mg of letrozole (26.3% vs 5.9%, P <.05). No multiple pregnancies occurred. Conclusion (s): Compared with the daily dose of 2.5 mg, 5 mg of letrozole is associated with more follicles and a higher pregnancy rate. It appears that 5 mg daily for 5 days is a preferred dose of letrozole for superovulation.