Objective: We have previously reported the advantages of a 2.5 mg dose of LE, a non-steroidal aromatase inhibitor in the treatment of patients with unexplained infertility, when compared to standard treatment with clomiphene citrate. In this study we investigated whether treatment with a higher dose of LE offers an advantage over treatment with a 2.5 mg dose in patients with unexplained infertility undergoing IUI treatment.Design: Prospective randomised double blind trial.Materials/Methods: Patients with unexplained infertility and in the absence of ovarian cysts on ultrasound scan performed between day 1 and 3 of the menstrual cycle, were offered to participate in the study. The patients were randomised in a two to one fashion, to receive either 2.5 mg or 5.0 mg of LE for five days starting from cycle day 3. Ultrasound scans and blood tests were performed on cycle day 7, and as required thereafter, until the dominant follicle reached 18mm in mean diameter. Ovulation was then triggered with 10,000 IU hCG and IUIs were performed 24 and 48 hours later. In subsequent cycles patients remained in their original study group. A maximum of three cycles were offered to each patient.Results: A total of 93 patients were recruited into the study. Of these 65 received 2.5mg of LE and 28 received 5mg of LE. There was no difference between the two groups in age (33.1 years vs. 31.9 years p = .17), duration of infertility ( 33 vs. 25 months p = .06), semen characteristics or any other relevant parameters. On day 7 of the menstrual cycle the group receiving 2.5mg of LE had significantly higher serum E2 (224 vs. 101 pmol/mL p = .001). On day of hCG administration E2 was higher in group receiving 2.5 mg of LE (623 vs. 419 pmol/mL p = .007), while more follicles >14mm in the mean diameter were recruited in the group receiving 5.0 mg of LE (2.0 vs. 1.58 p = .0.01). Endometrial development was not different between two goups. A marginally higher pregnancy rate was recorded in the group receiving 5.0mg of LE (17.9% vs 13.9%).Conclusions: Patients with unexplained infertility, when treated with a higher dose of LE develop more follicules without a detrimental effect on endometrial development. Therefore, perhaps, a 5mg dose of LE is more effective in ovulation induction in this group of patients.Supported by: none. Objective: We have previously reported the advantages of a 2.5 mg dose of LE, a non-steroidal aromatase inhibitor in the treatment of patients with unexplained infertility, when compared to standard treatment with clomiphene citrate. In this study we investigated whether treatment with a higher dose of LE offers an advantage over treatment with a 2.5 mg dose in patients with unexplained infertility undergoing IUI treatment. Design: Prospective randomised double blind trial. Materials/Methods: Patients with unexplained infertility and in the absence of ovarian cysts on ultrasound scan performed between day 1 and 3 of the menstrual cycle, were offered to participate in the study. The patients were randomised in a two to one fashion, to receive either 2.5 mg or 5.0 mg of LE for five days starting from cycle day 3. Ultrasound scans and blood tests were performed on cycle day 7, and as required thereafter, until the dominant follicle reached 18mm in mean diameter. Ovulation was then triggered with 10,000 IU hCG and IUIs were performed 24 and 48 hours later. In subsequent cycles patients remained in their original study group. A maximum of three cycles were offered to each patient. Results: A total of 93 patients were recruited into the study. Of these 65 received 2.5mg of LE and 28 received 5mg of LE. There was no difference between the two groups in age (33.1 years vs. 31.9 years p = .17), duration of infertility ( 33 vs. 25 months p = .06), semen characteristics or any other relevant parameters. On day 7 of the menstrual cycle the group receiving 2.5mg of LE had significantly higher serum E2 (224 vs. 101 pmol/mL p = .001). On day of hCG administration E2 was higher in group receiving 2.5 mg of LE (623 vs. 419 pmol/mL p = .007), while more follicles >14mm in the mean diameter were recruited in the group receiving 5.0 mg of LE (2.0 vs. 1.58 p = .0.01). Endometrial development was not different between two goups. A marginally higher pregnancy rate was recorded in the group receiving 5.0mg of LE (17.9% vs 13.9%). Conclusions: Patients with unexplained infertility, when treated with a higher dose of LE develop more follicules without a detrimental effect on endometrial development. Therefore, perhaps, a 5mg dose of LE is more effective in ovulation induction in this group of patients. Supported by: none.