The purpose of this study was to elucidate the long-term skeletal and dental stability after combined orthodontic and orthognathic surgical treatment of mandibular prognathism with the intraoral vertical ramus osteotomy (IVRO) as the surgical technique followed by 6 weeks of intermaxillary fixation (IMF). Thirty-six patients were included in the study. Mean age at surgery was 21.6 years. Lateral cephalograms and study casts obtained before the start of treatment (T0), and 8 weeks (T1), 1 year (T2), and 12.5 years (T3) after the operation were evaluated. Mean mandibular setback measured at point B was 8.3mm. Between T1 and T2, a mean anterior relapse of 12% of the setback was observed. Between T2 and T3, the anterior relapse persisted, but decreased to 7% of the setback measured at point B. Despite dental adjustments in both jaws, a statistically significant reduction in overjet was observed during both observation periods. However, the change of the mandible in an anterior direction was small and of minor clinical importance for most of the patients. In conclusion these results indicate that combined orthodontic and orthognathic surgical treatment of mandibular prognathism with the IVRO as the surgical procedure followed by 6 weeks of IMF provides predictable and good long-term clinical results.