Various techniques and modifications have been introduced in the treatment of mandibular prognathism. However, there are still few reports concerning long-term stability, especially using the intraoral vertical ramus osteotomy (IVRO) method. The purpose of this study was to investigate the long-term stability for correction of mandibular prognathism using IVRO. Twenty-five mandibular prognathism patients were treated by bilateral IVRO, and were evaluated cephalometrically by reference to the menton. A set of 3 standardized lateral cephalograms were obtained from each subject preoperatively (T1), immediately postoperatively (T2), and after 2 years postoperatively (T3). Relapse was defined as forward movement of menton after the 2-year follow-up. The mean setback of the menton was 12.8 mm in horizontal direction and 0.9 mm downward in vertical direction. The average follow-up was 33.9 months. The mean relapse was 1.3 mm (10.2% = 1.3 of 12.8) in forward direction and 0.6 mm in upward direction. There was no significant movement in the vertical direction. However, significant relapse was shown in the horizontal direction, even though the amount was small. The long-term stability of our present study suggested that IVRO is useful for correction of mandibular prognathism.