Segmental instability secondary to spinal degeneration is one of the major causes of adult low back pain and sciatica as well as herniated nucleus pulposus. However, the proper management of segmental instability has not yet been clearly determined. In addition, controversy exists regarding the need for decompression with/without fusion, the perferred approach method, anterior or posterior, and the use of instrumentation. The purpose of this study is to determine the proper treatment for different types of lumbar segmental instability. Thirty four patients with lumbar segmental instability who had operative treatment were subjected to clinical and radiological analysis. The lumbar segmental instability was classified into 5 types according to flexion and extension lateral X-ray, myelography and accompanying clinical symptoms. Type I (6 patients) was charcterized by stenosis in extension with angular difference of disc space on flexion and extension radiogram. Treatment included posterolateral fusion (PLF) 3 (patients), PLF with decompression (2 patients), and anterior interbody fusion (1 patients). Type II (6 patients) was stenosis in extension with retrolisthesis. Type Il A (2 patients) was instability which reducible in flexion accompanying with symptom relief, and they underwent PLF with pedicular screw fixation to maintain fusion in flexion. Type II B (4 patients) was also reducible in flexion but symptoms persist with stenotic dye column, and treated with PLF and decompression such as laminectomy, foraminotomy, and facetectomy (3 patients), pedicular screw fixation (1 patient). Type III was stenosis in flexion with anterior translation (19 patients). Type III A (3 patients) was reducible in extension accompanying with symptom relief and underwent PLF in extension with pedicular screw fixation (1 patient), or cast immobilization to maintain reduction until fusion (2 patients). Type III B (16 patients) was also reducible in extension but symptoms persist with stenotic dye column, and all the patients underwent decompressive surgery and PLF with pedicular screw fixation (6 patients), or cast immobilization to maintain reduction in extension (10 patients). Type IV (1 patient) was stenosis in flexion with retrolisthesis and underwent PLF, decompressive laminectomy and cast immobilization. Type V (2 patients) was lateral instability in side-bending and treated with decompressive lamilaminectomy, PLF and post-operative cast (1 patient), pedicular screw fixation (1 patient). All of patients were followed for more than post-operative one year except 2 patients (one type I, 1 month, type II, 2 months each). The results were graded as excellent, good, and poor. Type I (6 patients) had 4 excellent, 2 good, Type II (6 patients) had 3 excellent, 2 good, 1 poor, Type III (19 patients) had 11 excellent, 5 good, 3 poor, Type IV (1 patient) had good result and Type V (2 patients) had 1 good and 1 excellent result, respectively. All of 32 patients had a solid union at last follow-up. However, among the patients who had pedicular screw fixation in Type III (7 patients), 2 had collapse of the disc height, and 1 had screw failure. Those changes did not influence the clinical result. Conclusively, classification according to findings on flexion and extension lateral X-ray, myelography and accompanying clinical symptoms was useful to determine the method of surgical treatment for lumbar segmental instability. The pedicular screw fixation system with rigid rod was effective to maintain the lumbar spine in reduced position, and to maintain spinal canal and neural foramen wide. However, there was no significant difference in clinical result between pedicular screw fixation and post-operative cast immobilization cases.