In 30 adolescent patients with 36 structural curves undergoing posterior spinal arthrodesis using Cotrel-Dubousset (CD) instrumentation, computer tomography (CT) was performed to evaluate the change in rotation and in the sagittal plane. The average preoperative Cobb angle was 51.7 degrees, and improved to 18.7 degrees postoperatively, or 64.9%. The angle of rotation of the apical vertebra relative to the anterior midline of the body (RAML) improved from 27.9 degrees preoperatively to 21.5 degrees postoperatively (23.9%). The angle of rotation relative to the sagittal plane (RAsag) improved from 15.2 degrees preoperatively to 13.4 degrees postoperatively (14.3%). The kyphotic angle increased an average of 10.6 degrees in the 14 patients whose preoperative angle was less than 25 degrees. As measured by CT, CD instrumentation improves both the rotational and sagittal plane deformities better than that reported for other systems.