Objective. To evaluate pregnancy rates with letrozole and clomiphene citrate (CC) alone for superovulation in an intrauterine insemination program for women with recently surgically treated minimal to mild endometriosis. Design. A randomized controlled trial following the CONSORT criteria. Setting. University teaching hospital and a private practice setting. Patients. 136 women with primary infertility due to minimal to mild endometriosis who did not achieve pregnancy after six to 12 months following laparoscopic treatment. Interventions: Superovulation using 5mg letrozole/day (69 women, 220 cycles) or 100mg CC/day (67 women, 213 cycles) for five days combined with intrauterine insemination up to four cycles. Main outcome measures. Clinical pregnancy rate per cycle, cumulative pregnancy rate after four cycles, number of follicles, serum estradiol, endometrial thickness on the day of human chorionic gonadotropin administration, serum progesterone, miscarriage and live birth rates. Results. The clinical pregnancy rate per cycle and the cumulative pregnancy rate after four cycles were comparable (15.9 vs. 14.5% and 64.7 vs. 57.2%; p=0.82, p=0.71 in letrozole and CC groups, respectively). Two twin pregnancies occurred in the CC/intrauterine insemination group. Miscarriage and live birth rates were comparable (11.4 vs. 12.9% and 44.9 vs. 40.3%; p=0.47, p=0.62 in letrozole and CC groups, respectively). The total number of follicles and serum estradiol on the day of human chorionic gonadotropin administration were significantly increased in the CC group. Conclusions. Superovulation with letrozole is not more effective than clomiphene citrate alone in an intrauterine insemination program for women with minimal to mild endometriosis who did not achieve pregnancy after six to 12 months following laparoscopic treatment. ClinicalTrials.gov ID: NCT01334762.