Thoracolumbar idiopathic scoliosis usually is treated by anterior spinal fusion. However, short posterior spinal fusion that includes only the structural curve has been tried in a limited number of patients. The fusion may end cranially in the lower thoracic region and cause an increase in sagittal decompensation at the proximal junction. From July 1989 to July 1998, 14 patients were treated with thoracolumbar idiopathic scoliosis by short posterior spinal fusion. The lateral radiographs were evaluated preoperatively, immediately postoperative, and during followup. The focal kyphotic angle was used to examine the changes in focal sagittal alignment. A 10 degrees progression was defined as the radiographic criterion for the development of junctional kyphosis. Proximal junctional kyphosis occurred in six of the 14 patients, in which one patient needed revision surgery. In all six patients, the average preoperative lumbar lordosis was greater than 35 degrees, and decreased more than 10 degrees during surgery. In the five patients with a focal kyphotic angle larger than 10 degrees, four had proximal junctional kyphosis develop. According to the current findings, short posterior spinal fusion can be done only if the focal kyphotic angle proximal to the fusion is less than 10 degrees, and the lumbar lordosis must be preserved carefully during surgery.