Abstract: Accuracy assessment of the CASSOS 2001—computer-assisted simulation system for orthognathic surgery SONG En, MO Kang-nan, ZHU Fu-dong, GE Wei-li, LI Shenglai, XIE Zhi-jian.(Department of Oral and Maxillofacial Surgery, Stomatological Hospital, School of Medicine, Zhejiang University, Hangzhou 310006, China) Abstract：　Objective　To assess the accuracy of the computer-assisted simulation system for orthognathic surgery. Methods　The study consisted of 25 patients with maxillofacial deformity treated with orthognathic surgery. Their lateral cephalograms and color lateral photos of soft tissue before surgery were input to the CASSOS 2001 system. Carry out cephalometric analysis and simulative orthognathic surgery, and predict occlusal relationship and lateral features of soft tissue. Then input the lateral cephalograms after surgery into the CASSOS 2001 system, and carry out cephalometric analysis. Choose ten parameters and compare predicted surgical parameters with actual postsurgical parameters. Results　Results of four parameters of hard tissue and five parameters of soft tissue in this study were similar. There were statistical differences in only one parameter of soft tissue, the labral length. Conclusion　CASSOS 2001 system is precise for the simulation of changes of hard tissue in orthognathic surgery. It has a little shortcoming in the simulation of soft tissue, especially in the prediction of lips. The positioning error of the marking points, and the soft to hard tissue movement proportion have a large influence in the prediction. CASSOS 2001 system still has great room for improvement.