The purpose of the study is to estimate hard and soft tissue changes after orthognathic surgery for the correction of the mandibular prognathism and to describe interrelationship and ratios of soft and hard tissue changes. The presurgical and postsurgical lateral cephalograms of 31 treated patients(17 males and 14 females) was used ; these patients had received combined orthodontic-surgical treatment by means of a bilateral sagittal split ramus osteotomy. Their ages ranged from 16 to 31 years and mean age was 21.4 years. A computerized cephalometric appraisal was developed and used to analyse linear and angular changes of skeletal and soft tissue profile. The statistical elaboration of the data was made by means of SPSS/PC⁺. The results of the study were as follows : 1. The correlations of soft and hard tissue horizontal changes were significantly high and the ratios were 97% at LI, 107% at ILS, and 93% at Pog´. 2. The correlations of vertical changes at Stm, LI and horizontal changes at Pog were high (26 %) and at the other areas were not statistically high. 3. The correlations of soft ad hard tissue vertical changes were not significantly high in all areas except Gn´ (30%) and Me´ (56%). 4. The soft tissue thickness was significantly decreased in upper lip and increased in lower lip, and the amount of changes after surgery was reversely correlated with initial thickness. 5. The facial convexity was increased and relative protrusion of upper lip was increased and that of lower lip was decreased. 6. The upper to lower facial height(G1-Sn/Sn-Me´) was increased and upper to lower jaw height(Sn-Stms/Stmi-Me´) was increased.