The aim of this study was to determine changes in the cross-sectional area of the lower lip and soft tissue change t o hard tissue change ratios subsequent to mandibular advancement surgery. Pretreatment (T1), presurgical (T2), and posttreatment (T3) lateral cephalograms of 27 patients who had undergone mandibular advancement surgery utilizing sagittal split osteotomies and rigid internal fixationwere digitized. A custom compute rprogram was used to calculate the cross-sectional area of the lower lip. Mean soft tissue change to hard tissue change ratios were computed for a number of variables. No statistically significant difference existed for the cross-sectional area of the lower lip from T1 to T2 or T1 to T3. A statistically significant difference existed for the lip area from T2 to T3, with an average decrease of 7.7 mm2 or 3.5%. Although statistically significant, the clinical significance of the 3.7% mean decrease in the area T2 to T3 is questionable. The cross-sectional area of the lower lip may be a useful variable for improving the accuracy of algorithms used to predict the response of the lower lip following mandibular advancement surgery.