Postoperative follow-up and multiple regression analysis of skeletal relapse following mandibular setback were carried out to clarify the timing and causes of the relapse. The subjects were 24 mandibular prognathism patients. All patients underwent intraoral oblique sagittal splitting osteotomy with circumferential wiring and intermaxillary fixation for 8 weeks. Occlusal splints were not used. Postoperative positional changes of segments were evaluated by lateral cephalograms taken at appropriate intervals. Horizontal relapse was most evident within six months after surgery; vertical relapse seldom occurred. Multiple regression analysis revealed little association between preoperative morphological patterns and postoperative relapse. Although spatial changes of the proximal segment at operation and age of the patient were the best predictors for postoperative horizontal relapse, analysis indicated unsatisfactory prediction of vertical relapse because of its rare occurrence. Based on these results, the aetiology of relapse is discussed and two proposals are suggested for its prevention.