Objective: To find out whether a single-administered GnRH agonist improves the live birth rate in real-life patients undergoing intrauterine insemination (IUI) cycles. Study Design: A prospective, randomized controlled trial in a public single tertiary center in Tampere University Hospital, Finland. Altogether 251 IUI cycles in 163 patients were randomized to triptorelin and a control group between January 2017 and April 2019. In the triptorelin group, the participants had a single administration of a subcutaneous GnRH agonist triptorelin 0.1 mg at the time of implantation. In the control group, there was no luteal phase support. The primary outcome measure was the live birth rate (LBR). The secondary outcome measures were clinical pregnancy rate (CPR) and miscarriage rate. Results: Overall, the live birth rate was lower in the triptorelin group compared to the control group (7.9 vs. 12.1%; p =.297). The clinical pregnancy rates were 12.6 and 13.7%, respectively. There were 2.4% miscarriages in the triptorelin group and no miscarriages in the control group. Ovarian stimulation with letrozole was associated with lower LBR among the triptorelin group, in comparison to the control group (0 vs. 14.7%, p =.020). In contrast, when gonadotrophin was added to the letrozole, LBR was almost doubled compared to the control group (15.9 vs. 8.3%, p =.341). Conclusion: A single administration of GnRH agonist in the luteal phase does not improve LBR in IUI cycles.