Objective: To evaluate the efficacy of fallopian sperm perfusion (FSP) using a new method similar to the FAST system® in comparison with standard intrauterine insemination (IUI) in patients with unexplained infertility. Design: Prospective, randomized, controlled study. Setting: Assisted conception service in a University Hospital. Patient(s): Women with unexplained infertility undergoing controlled ovarian hyperstimulation (COH). Intervention(s): After hCG administration, patients were randomized to either standard IUI or FSP. The women received the same treatment in the first and all subsequent cycles. A maximum of three cycles was performed. Intrauterine insemination was performed using a standard method, and fallopian sperm perfusion was performed using a commercial device for hysterosalpingography and tubal hydropertubation. Main Outcome Measure(s): Clinical and ongoing pregnancy rates. Result(s): A total of 132 cycles was completed: 66 IUI cycles and 66 FSP cycles. In the IUI group, there were 5 ongoing pregnancies, giving a pregnancy rate of 7.6 per cycle and 15.6% per patient; in the FSP group, 14 ongoing pregnancies occurred, giving a pregnancy rate of 21.2% per cycle and 42.4% per patient. The prevalence of multiple pregnancies, miscarriages and ectopic pregnancies was similar in the two insemination groups. Fallopian sperm perfusion was easy to perform, and no case of sperm reflux was observed. The procedure was well tolerated and no complications were observed. The costs were comparable with standard IUI. Conclusion(s): In the treatment of couples with unexplained infertility, the method for fallopian sperm perfusion described yields higher pregnancy rates than IUI, with no significant increase in costs or complications. However, these results need to be confirmed in larger studies before replacing IUI with FSP as standard practice.