Objective: To compare the efficacy of GnRH antagonist vs. GnRH agonist adminis tration for controlled ovarian hyperstimulation (COH) in assisted reproduction. Design: A prospective, randomized trial. Setting: Clinical research unit at a te rtiary care medical center. Patient(s): Sixty-five patients with unexplained in fertility or mild male subfertility undergoing COH for IUI. Intervention(s): Twe nty-nine women (group A) were randomized to receive 600 μg of busereline aceta te per day starting in the midluteal phase of the cycle (long protocol), whereas 36 women (group B) were treated with 0.25 mg/d of the GnRH antagonist Cetroreli x starting from day 6 of the cycle. The starting dose of recombinant FSH was 150 IU in women of both groups. Insemination was performed 34 hours after hCG injec tion. Main Outcome Measure(s): Clinical and successful ongoing pregnancy rate (P R), measurements of serum FSH, LH, E2, and P, number of recruited follicles, dur ation of stimulation period, and amount of gonadotropins used. Result(s): Women in group A required significantly more days of treatment (median: 12.0 vs. 9.0) and significantly more total units of recombinant FSH (median 1,800 vs. 1,550) a s compared with the corresponding values of the antagonist group (group B). Seru m FSH, LH, E2, and P were significantly higher on the antagonist group on days 2 and 6 of stimulation. However, these differences regress until the day of hCG a dministration. Conclusion(s): The GnRH antagonists have facilitated short and si mple treatment, and are particularly attractive for administration in women unde rgoing COH, achieving comparable PR with the long protocol regimen. Objective: To compare the efficacy of GnRH antagonist vs. GnRH agonist adminis tration for controlled ovarian hyperstimulation (COH) in assisted reproduction. Design: A prospective, randomized trial. Setting: Clinical research unit at a te rtiary care medical center. Patient (s : Sixty-five patients with unexplained in fertility or mild male subfertility undergoing COH for IUI. Intervention (s): Twe nty-nine women (group A) were randomized to receive 600 μg of busereline aceta te per day starting in the midluteal phase of the cycle (long protocol), while 36 women (group B) were treated with 0.25 mg / d of the GnRH antagonist Cetroreli x starting from day 6 of the cycle. The starting dose of recombinant FSH was 150 IU in women of both groups Main Outcome Measure (s): Clinical and successful & lt; / RTI & gt; pregnancy rate (PR), measurements of serum FSH, LH, E2, and P, number of recruited follicles, durtion of stimulation period , and am Out of gonadotropins used. Result (s): Women in group A required significantly more days of treatment (median: 12.0 vs. 9.0) and significantly more total units of recombinant FSH (median 1,800 vs. 1,550) as compared with the corresponding values ​​of The antagonist group (group B). Seru m FSH, LH, E2, and P were significantly higher on the antagonist group on days 2 and 6 of stimulation. However, these differences regress until the day of hCG a dministration. Conclusion (s) : The GnRH antagonists have facilitated short and si mple treatment, and are particularly attractive for administration in women unde rgoing COH, achieving comparable PR with the long protocol regimen.