Forty-eight patients with idiopathic scoliosis underwent posterior spinal fusion with Cotrel-Dubousset instrumentation (CDI). Each patient was given preoperative and postoperative pulmonary function tests (PFTs). Pulmonary volume improved a mean 0.40 L (16%), and pulmonary flow improved a mean 0.33 L (15%). Differences between preoperative and postoperative PFT values were shown to be statistically significant and correlated well with coronal side-bending correction. Twenty patients had preoperative and postoperative CT scans through the apical vertebra. Vertebral rotation was assessed on CT scan by the method of Aaro and Dahlborn (1,2). At the apex, the mean percentage improvements in the longitudinal axis rotation relative to the midline (16%) and to the sagittal plane (10%), the rib hump index (8%), and the kyphosis-lordosis index (0%) were minimal. Radiographically, the 66% mean improvement in apical vertebral translation was more substantial and consistent than the 10% mean improvement in apical vertebral rotation. Therefore, at the apex the CDI "derotation maneuver" may be more of a "translational maneuver."