Objective To compare a short and long interval between hCG administra tion and IUI after superovulation for the treatment of infertility. Design Prospective, r andomized clinical trial. Setting University hospital-based fertility clinic. P atient(s) Patients planning superovulation and IUI for the treatment of infertil ity. Intervention(s) Patients with ≥2 years of infertility enrolled for superov ulation and IUI treatment were randomized to IUI after a short (32-34-hour) or long (38-40-hour) interval after hCG injection. Superovulation was accomplish ed with hMG or recombinant FSH, with dose adjustment until the maturation of two to five follicles, at which time hCG was given. Sperm was prepared with a gradi ent centrifugation technique, with IUI performed high up in the uterine fundus. Main outcome measure(s) Pregnancy rates. Result(s) Of the 348 patient cycles randomized, 270 treatment cycles were initiated. Eighty -one initiated cycles were canceled, leaving 189 completed randomized cycles fr om 75 patients for analysis. Pregnancy rates were not significantly different be tween groups. There were pregnancies in 20 of the 96 short hCG-IUI interval cyc les (21%) and in 14 of the 93 long hCG-IUI interval cycles (15%) (odds ratio = 0.673, 95%confidence interval 0.297-1.518). Conclusion( s) Pregnancy rates a re the same after superovulation therapy whether IUI is done after a short or a long interval after hCG injection. Objective To compare a short and long interval between hCG administra tion and IUI after superovulation for the treatment of infertility. Design Prospective, r andomized clinical trial. Setting University hospital-based fertility clinic. P atient (s) Patients planning superovulation and IUI for the treatment of infertil ity. Intervention (s) Patients with ≥2 years of infertility enrolled for superovulation and IUI treatment were randomized to IUI after a short (32-34-hour) or long (38-40-hour) interval after hCG injection . Superovulation was done with hMG or recombinant FSH, with dose adjustment until the maturation of two to five follicles, at which time hCG was given. Sperm was prepared with a gradi ent centrifugation technique, with IUI performed high up in the uterine fundus. Eighty-one initiated cycles were canceled, leaving 189 completed randomi There were pregnancies in 20 of the 96 hCG-IUI interval cyc les (21%) and in 14 of the 93 long hCG-IUI interval cycles (15%) (odds ratio = 0.673, 95% confidence interval 0.297-1.518). Conclusion (s) Pregnancy rates a re the same after superovulation therapy whether IUI is done after a short or a long interval after hCG injection.