In the surgical trestment of unstable Iumbar spinal disorders, various methods of instrumentstion have been used for stabilization of spine, correction of deformity and reduction of fracture, despite of their technical demands. This paper reports the results of 30 patients of Cotrel-Dubousset instrumentation and posterolateral fusion for the lumbar spinal disorders associated with instability. The results obtained were as follows:l. Of the 30 cases, males were 15 cases(50%), females were 15 cases(50%) and average age of the patients was 42 yesrs ranging from 16 to 65 years. 2. Types of the disorders were spondylolisthesis in 16, failed back syndrome in 6, spondylolysis in 2, spinal canal stenosis in 2, scoliosis associated with degenerative spondylitis in 2, and burst fracture in 2 cases. 3. Among the 28 cases, 11 cases were excellent and 14 cases were good in Steffee's criteria (2 cases of burst fractures were excluded). 4. In spondylolisthesis, the average percentage of slippage was changed from 20% preoperatively to 5% postoperatively and 5.2% at the final follow-up. The initial correction rate was 75% and the loss of correction was not significant in follow-up study. 5. The patients with neurological claudication were permitted ambulation from postoperative 3rd day. In the final follow-up, the 8 patients(80%) could walk for more than 1 hour without pain. 6. The complications were relatively few(3 cases of hematoma, 2 cases of marginal sloughing of skin with superficial infection and one case of transient dysuria). In this study, this method seemed to be superior to others for the treatment of lumbar spinal disorders associated with instability especially degenerative spinal disorders in the respect of stability. Simultaneously, it provides immediate postoperative rehabilitation without rigid external support.