The spinal fusion is used for correcting malformations, stabilizing unstable segments, and suppressing the progression of disease, but it causes more load on the adjacent segment and as a result, the degenerative changes accelerate. Furthermore, the accelerated changes case spinal stenosis, degradation of nucleus pulposus, degenerative spondylolisthesis, acquired isthmus defect, and arthritis of articular facet, it rarely needs surgical treatment. The purpose of this study is to know the effect of the fusion level and range on the adjacent segment, to be a standard for determining the fusion range prior to operation, and to help to find the complication like degenerative changes by measuring the range of motion on the adjacent segment after the spinal fusion. Authors analyzed the result of 29 cases of posterolateral lumbar spinal fusion which were operated from February 1989 and January 1994. The intervertebral angle was measured on the flexion and extension lateral radiographs, and the calculated angular motion and sagittal plane rotation at follow-up periods were compared with those of preoperative values. The results were as follows; 1. Among the adjacent segm0ent, the increment of angular motion of superior adjacent segment was 2.6°(-5°~15°) which was higher than that of inferior one with 1.5°(-3°~7°) 2. The increment of angular motion of superior adjacent segment for 1 segment rather than 2 or 3 segments was more than two times with 3.9°(-2°~15°) 3. Regardless of level and range of fusion, the increment of angular motion was 3.0°(-2°~8°) on 4.5th lumbar segment which was increased mostly and 1.5°(-3°~7°) on 5th lumbar and 1st sacral segment which was increased leastly. 4. There were 3 cases showed instability at the superior adjacent segment and no cases showed instability at the inferior adjacent segment among all the cases showing stability prior to operations. In conclusion, the sagittal plane rotation was increased in adjacent segment after posterolateral spine fusion. So, we would recommend that the adjacent segment is very important to the decision of fusion extent and level.