Objective: To prospectively compare pregnancy rates for couples undergoing IUI by a single healthcare provider with random assignment to either rigid (Tomcat; Kendall Sovereign, Mansfield, MA) or flexible (Soft Pass; Cook, Spencer, IN) ca theters. Design: Randomized, controlled study. Setting: Tertiary care infertilit y practice. Patient(s): One hundred infertile women enrolled from a single infer tility practice who met the inclusion and exclusion criteria for IUI were enroll ed. Of the 100 patients, 95 underwent ovarian stimulation before IUI: 35 with cl omiphene citrate alone, 60 with clomiphene citrate combined with a single dose o f gonadotropins. Intervention(s): Timed IUI was performed with either rigid (Tom cat) or flexible (Cook) insemination catheters. Main Outcome Measure(s): Pregnan cy rate per cycle. Result(s): No differences were noted between groups for the f ollowing: age, length of infertility, day-3 FSH level, number of prior IUI cycl es, total motile sperm count, days of abstinence, infertility diagnoses, insemin ation ratings, and stimulation protocols. No difference in pregnancy rates was o bserved between rigid and flexible catheter groups. Conclusion(s): There is no s tatistically significant difference between flexible and rigid catheters for IUI . Objective: To prospectively compare pregnancy rates for couples undergoing IUI by a single healthcare provider with random assignment to either rigid (Tomcat; Kendall Sovereign, Mansfield, MA) or flexible (Soft Pass; Cook, Spencer, IN) ca theters. Design: Randomized Patient (s): One hundred infertile women enrolled from a single infer tility practice who met the inclusion and exclusion criteria for IUI were enroll ed. Of the 100 patients, 95 underwent ovarian stimulation before IUI: 35 with cl omiphene citrate alone, 60 with clomiphene citrate combined with a single dose of gonadotropins. Intervention (s): Timed IUI was performed with either rigid (Tom cat) or flexible (Cook) insemination catheters. Main Outcome Measure ( Result (s): No differences were noted between groups for the f ollowing: age, length of infertility, day-3 FSH level, number of prior IUI cycl es, total motile sperm count, day s of abstinence, infertility diagnoses, inseminacy ratings, and stimulation protocols. No difference in pregnancy rates was o bserved between rigid and flexible catheter groups. Conclusion (s): There is no tattoo significant difference between flexible and rigid catheters for IUI.