The purpose of this study was to evaluate the three-dimensional change of the lip morphology on lip closure and at smiling in patients with mandibular prognathism accompanied by mandibular deviation before and after surgical orthodontic treatment. Twelve patients (6 females, 6 males; age 31.3&plusmn;13.2 years) were selected for this study. Frontal and lateral cephalograms and three-dimensional (3D) images were taken before and after the treatment. The inclusion criteria of used cases were as follows: 1) ANB angle smaller than 0&deg;, 2) having negative overjet, 3) more than 5mm of lateral deviation between the mandibular midline (represented by Menton) and the facial midline, 4) less than 2&deg; maxillary roll, 5) bilateral sagittal split ramus osteotomy performed, 6) normal SNA angle, and 7) without any congenital anomalies including cleft lip and/or palate.<br>Cephalometric analysis showed that the pre-treatment ANB angle was &minus;1.92&plusmn;1.18&deg;, and the amount of deviation of Me was 8.16&plusmn;2.83mm. The average setback movement of the mandible was 0.70&plusmn;5.00mm on the deviated side and 8.61&plusmn;2.84mm on the non-deviated side. The post-treatment ANB angle was 1.61&plusmn;1.26&deg;, and the post-treatment deviation of the Me was 1.58&plusmn;1.80mm.<br>The 3D facial images showed that there was a clear difference in the positions between the left and right chelions at the time of lip closure and smiling at pre-treatment, and both chelions were almost symmetrically positioned at post-treatment. When comparing the 3D Cartesian coordinates between pre-and post-treatment, there were significant differences in chelion on the non-deviated side at the time of both lip closure and smiling. While the upper lip point moved inferiorly at the time of lip closure and smiling, the lower lip point showed remarkable changes, especially in the horizontal and anteroposterior directions.<br>The asymmetry of lower lip surface area at pre-treatment was corrected by surgical orthodontic treatment both at the time of lip closure and smiling at post-treatment. Lip asymmetry was corrected by surgical orthodontic treatment not only at the time of lip closure but also at smiling due to three-dimensional changes of the non-deviated side of the chelion.