Preoperative and postoperative TMJ symptoms were observed in mandibular prognathism of 30 patients operated on by Le Fort I osteotomy and intraoral vertical ramus osteotomy. The alterations of TMJ symptoms were investigated and the relationship between changes of TMJ symptoms and some cephalometric values including occlusal plane angle, mandibular plane angle, posteior ramal height and a degree of deviation of mandibular incisor midline to facial midline were observed. The results are as follows. The incidence of patient with TMJ symptoms before orthognathic surgery was 40% and after orthognathic surgery was 20%. The most frequetly encountered symptoms in orthognathic TMJ dysfunction patients were TMJ pain and/or clicking. After surgery 66% of the preoperative symptomatic patients reported improvement TMJ symptoms. On the other hand 2 patients (6%) of the preoperative asymptomatic patients developed TMJ symptoms after surgery. Preoperatively, 60% of the facial asymptomatic patients with mandibular prognathism had TMJ symptoms. The more severe facial asymmetry was, the higher incidence of TMJ symptoms was. The alteration of occlusal plane angle and mandibular plane angle seems to be one of the contributing factors which make to change TMJ symptoms in orthognathic patients. But its amount seems to be low significance. Increase or decrease of posterior ramal height have influence on the change of TMJ symptoms.