The aim of the spine fusion is to obtain the stability of the spine. Spinal fusion have been used for the treatment of tuberculosis and arrest of the progress of spinal deformity such as scoliosis. Nowadays the spine fusion is used often in conditions which present the instability; spondylosis, spondylolisthesis and the unstable postlaminectomy spine. Unilateral posterolateral fusion plus hemiposterior fusion were done only in the cases who had wide hemilaminectomy, while in rest of cases bilateral posterolateral fusion was indicated in this series. Fifty-two cases, treated with posterolateral lumber fusion during the period from June 1980 to Dec. 1985, were analysed clinically and radiologically and the following results were obtained. l. Among the 52 cases, 25 cases(48.1%) were male; and 27 cases(51.9%) were female; The youngest was 16 years of age and the oldest one was 63 years of age. Average age of the patient was 39.6 years. 2. Fifteen patients had spinal stenosis, 14 cases of spondylolisthesis, 11 cases of H.N.P., 6 cases of spondylolysis, 4 cases of tuberculous spondylitis and 2 cases of fracture-dislocation of lumbar spine. 3. As a method immobilization after operation, postoperatively hips spica cast was applied in 4 cases for 12 weeks, Norton-Brown back brace in 9 cases following 6 weeks of posloperative hip spica cast immobilization and Norton-Brown brace 39 cases following 2 weeks of postoperative bed rest. 4. In 41 cases bilateral posterolateral fusion was done, and in 11 cases unilateral posterolateral fusion plus hemiposterior fusion were done. The bony union was established within 4 months after bilateral posterolateral fusion in 37 cases and after unilateral posterolateral fusion in 9 cases. In 2 cases complete union was obtained within 6 months after fusion. 5. The union rate was 92.7% in bilateral posterolateral fusion and 90.9% in unilateral posterolateral fusion. There was no significant differences of the union rate between bilateral and unilateral fusion groups. Therefore, the unilateral posterolateral fusion is a suitable method in the treatment of mild spinal instability regardless of its causes. The procedure has benefits, such as short operation time, less blood loss, and less surgical burden to the patient.