A study was done on the role of postoperative leg length inequality (LLI) in aseptic loosening after total hip arthroplasty (THA). Data on 405 McKee-Farrar hip replacements performed at the Invalid Foundation, Helsinki, Finland were available for the study. The mean lengthening of the operated leg was 0.8 cm after an average follow-up of 7.7 years. Of these hips, 63 (15.6%) subsequently underwent revision due to aseptic loosening of the prosthesis. Several risk factors were analysed using multivariate stepwise regression analysis to find factors which predispose patients to loosening: overlength of the operated limb proved to be the most important. Our conclusion is that overlengthening should be avoided by careful preoperative and intraoperative length measurements and proper selection of prostheses. If there is marked overlengthening of the replaced hip postoperatively, it should be corrected.