OBJECTIVES To investigate the controversy whether an increased endometrial thickness has an effect on pregnancy, implantation, or abortion rates in in vitro fertilization-embryo transfer (IVF-ET) cycles. DESIGN Retrospective analysis. SETTING A university-based IVF center. PATIENT(S) Five hundred seventy women under the age of 40. INTERVENTION(S) Measurements of endometrial thickness on day of human chorionic gonadotropin (hCG) administration. Cycles were compared by endometrial thickness of </=14 mm in 510 women to a thickness of >14 mm in 60 women. MAIN OUTCOME MEASURE(S) Implantation, pregnancy, and abortion rates. RESULT(S) Implantation, pregnancy, and abortion rates were similar in each group. In cycles where the endometrial thickness was </=14 mm, the rates were 20.9%, 43.1%, and 11.8% compared with 25.5%, 48.3%, and 13.8% in cycles >14 mm. CONCLUSION(S) No adverse effects of a thickened endometrium were demonstrated on implantation, pregnancy, or abortion rates in the first IVF-ET cycle. These findings fail to corroborate with those of Weissman et al. and support those of Yakin et al.