Objective: To determine and compare the transvaginal ultrasonographic (US) endometrial changes immediately after IUI using the Edwards Wallace (H.G. Wallace, Limited, Colchester, Essex, UK) and Tom-Cat (Sherwood Medical, St. Louis, MO) catheters. Design: Prospective study. Setting: IVF unit. Patient(s) and Intervention(s): Eighty-two infertile patients underwent 112 cycles of ovulation induction with IUI. Either the Edwards Wallace catheter (group 1, n = 32) or the Tom-Cat catheter (group 2, n = 80) was used for sperm insemination. The presence of an endometrial three-layer pattern before IUI was a prerequisite for inclusion in the study. After each IUI, the endometrium was reassessed by transvaginal US. Main Outcome Measure(s): Ultrasonographic endometrial changes, clinical pregnancy rates (PRs), complications, and patients' complaints were compared between the two groups. Result(s): Total destruction of the endometrial three-layer pattern was observed in 12.5% of the cycles in group 1, compared with 50% of the cycles in group 2. Clinical pregnancies occurred in 14 (12.5%) of the 112 IUI cycles. A higher PR was achieved when the endometrial three-layer pattern was preserved after IUI. The patients in group 2 had more complaints of bleeding and pain during the procedure. Conclusion(s): Ultrasonographic changes after IUI suggest that the Edwards Wallace catheter is significantly less traumatic to the endometrium than the Tom-Cat catheter. Although both catheters yielded the same overall PR, there was a trend indicating that sparing the endometrial three-layer pattern from damage increases the chance of conception. (C) 1997 by American Society for Reproductive Medicine.