The degenerative spondylolisthesis is one of the most common causes of the prominent central and recess stenosis which are produced by the hypertrophy of the facet joints and anterior slipping of the posterior arch. The resulting neurogenic symptoms in the legs are the major causes of the surgical treatment in the degenerative spondylolisthesis and the complete decompression is indicated for these types of spinal stenosis. The decompression procedures performed in the degenerative spondylolisthesis makes more unstable and induce the late instability and the post - operative instability is usually concerned in the surgical treatment of this problem. So it is mandatory to define the exact causes of the nerve entrapment and have proper surgical decompression without late instability of the degenerative spondylolisthesis. Authors reviewed the findings of the pre - operative evaluatian and the surgical findings of 18 cases of the degenerative spondylolisthesis which were managed by the surgical decompression for their spinal stenosis at the Department of Orthopaedic Surgery, Fatima hospital, Daegu, Korea for 2 years from June of 1986 and were able to define the causes of the nerve entrapment and to suggest an adequate and least amount of decompression in each causes. The results obtained were as follows: 1. The 4th lumbar vertebrae were slipped anteriorly and the facet joints revealed advanced degenerative changes in all cases. 2. The central stenosis was found in 7 cases and the major cause of nerve entrapment was hypertrophy of the facet joints and the anterior slip of the posterior arch was another cause. 3. The lateral recess stenosis entraping the 5th lumbar nerve roots was found in all cases and the causes of nerve entrapment were due to the anterior slip of the inferior articular process of 4th lumbar vertebrae in 4 cases, the hypertrophy of the superior articular process of 5th lumbsr vertebrae in 9 cases and both of the above in 5 cases with the latter being major cause. 4. There was no intepvertebral foraminal stenosis entraping the 4th lumbar nerve root.5. The procedures of surgical decompression were discussed for each causes of the nerve entrapments.