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Objective: To compare the efficacy of single-dose letrozole (25 mg) with a 5-day course (5 mg/day) for ovulation induction (OI). Design: Retrospective cohort study. Setting: Hospital. Patient(s): Patients undergoing first round of OI and intrauterine insemination with letrozole from January 2015 through December 2017. Intervention(s): Patients received letrozole as either a single 25 mg dose for 1 day (1D) versus 5 mg daily for 5 days (5D). A secondary analysis was performed on patients also receiving gonadotropins (GND). Main Outcome Measure(s): Pregnancy rate (PR) determined by positive human chorionic GND. Result(s): There were 847 patients included in the study, 302 in the 1D group and 284 in the 5D group; 261 patients had concurrent GND administration, 162 1D+GND and 99 5D+GND. There was no difference in smoking status, primary versus secondary infertility, or total motile sperm concentration. Comparing 1D with 5D, there was a statistically significant, although not clinically relevant, difference in both age and body mass index (31 vs. 31.8 years; 26.2 vs. 27.4, respectively). Similarly, comparing 1D+GND with 5D+GND, there was statistically significant difference in body mass index (27.19 vs. 29.1). Secondary outcomes included live birth rate (LBR), multiple gestation rate (MG), and miscarriage rate (SAB). There were no differences between 1D and 5D in the primary outcome of PR (14.2% vs. 11.6%), LBR (9.6% vs. 7%), MG (16.2% vs. 13.8%), or SAB (16.22% vs. 13.8%). In looking at the GND groups alone, there was no difference in PR (18.3% vs. 23.8%), LBR (11.72% vs. 17.86%), MG (8.7% vs. 5.56%), or SAB (13.64% vs. 5.56%). There was a significant difference in cycle cancellation rate in the 1D versus 5D groups (3.9% vs. 9.6%); however, this was not seen in the 1D+GND versus 5D+GND groups. Conclusion(s): A single-dose protocol with letrozole in an OI/intrauterine insemination cycle may be considered an alternative to standard 5D dosing protocols with the potential for improved compliance and similar reproductive outcomes.