FIFTY-TWO PATIENTS WITH unstable degenerative spondylolisthesis treated with the “AO internal fixator” and posterolateral fusion were reviewed. The major purpose of this study is to observe whether this pedicle fixation system could adequately decompress the nervous system tissue by the restoration of the spinal canal and, hence, replace the conventional decompressive laminectomy for the treatment of this disease entity. The results were satisfactory, showing that 92% of the patients with radicular pain, 89% of the patients with low back pain, and 86% of the patients with intermittent claudication improved postoperatively. Observing the results, only two groups of patients with unstable degenerative spondylolisthesis are not suitable for this treatment modality. The first group consists of those patients who have a spondylolisthesis with borderline instability. The second group consists of those patients who have a positive Lasèque's sign.