The purpose of this retrospective study was to analyze the accuracy of computer predictions by CASSOS (Computer-Assisted Simulation System for Orthognathic Surgery) 2001 software (2000 SoftEnable Technology). Forty adult patients who had undergone orthognathic surgery were evaluated. Pre- and postsurgical lateral cephalographs were scanned into the computer, and 71 landmarks for each cephalograph were digitized. Digitization error was assessed from repeated digitizations. A customized cephalometric analysis consisting of 14 measurements was used in this study. Predicted and actual postsurgical hard tissue landmarks were compared using the Student t test. Results showed good correlation between repeated digitization for all measurements. There were no statistically significant differences in 10 of the 14 measurements. The differences that were statistically significant were in angular measurements for SNA angle, upper incisor to maxillary plane angle (U1-MxP), interincisal angle (U1-L1), and upper incisor to anterior cranial base angle (U1-SN). The greatest mean difference measured was the interincisal angle (U1-L1) which, although statistically significant, was clinically insignificant. This investigation showed that CASSOS 2001 software provides accurate hard tissue prediction for orthognathic surgical procedures.