['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.', 'Retrospective analysis.', 'A university-based IVF center.', 'Five hundred seventy women under the age of 40.', '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.', 'Implantation, pregnancy, and abortion rates.', '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.'] Does increased endometrial thickness on the day of human chorionic gonadotropin injection adversely affect pregnancy or implantation rates following in vitro fertilization-embryo transfer?