METHODS A study was done to evaluate the use of voluntary supine side bending radiographs and Risser table traction radiographs in adolescent patients undergoing posterior spinal fusion for idiopathic scoliosis. OBJECTIVE To compare the usefulness of supine side bending and traction radiographs in assessing curve flexibility and determining fusion levels in patients undergoing posterior spinal fusion for adolescent idiopathic scoliosis. BACKGROUND Supine side bending radiographs have been used in the preoperative evaluation of idiopathic scoliosis to determine curve flexibility and fusion area. Traction films have been used to determine the flexibility of large curves and neuromuscular curves where active side bending is not possible. No study to date has compared the use of these films in patients with adolescent idiopathic scoliosis undergoing surgery. METHODS Seventy-five patients with more than a 2-year follow-up period after surgery were included in this study. Preoperative radiographs included a standing posteroanterior and lateral film and both supine maximal voluntary side bending films and a traction film done on a Risser table. A preoperative review of these radiographs was done to determine curve flexibility and fusion levels. At follow-up evaluation, the patients were examined for any evidence of decompensation or "adding-on" of levels. RESULTS For curves less than 60 degrees, side bending radiographs showed greater curve correction than traction radiographs, whereas the opposite was true for curves greater than 60 degrees. For King I and II curves, side bending radiographs were superior for determination of lumbar curve flexibility and for distinguishing these two types of curves. On traction radiographs, the stable vertebra was 1.4 vertebral levels higher than on the standing film. When the fusion level was moved proximally because of the traction radiograph, decompensation or "adding-on" commonly occurred. CONCLUSIONS Supine bending radiographs are superior to traction radiographs for assessing curve flexibility except for curves more than 60 degrees. The selection of the distal extent of fusion based on the traction radiograph gave a large number of poor results. The selection of fusion levels in adolescent Idiopathic scoliosis is best determined by a combination of standing posteroanterior and lateral radiographs and the supine maximum voluntary bend films.