The instability of the degenerative spondylolisthetic lumbar spine was clinically evaluated by patients symptoms and serial radiograms. With X-ray films of patients lumbar spine, the lumbosacral angle, the sacral angle, the vertebral height difference between anterior and posterior surface of the 5th lumbar vertebra and the level of intercristal line were analysed. And the results of treatment were analysed too. The Results were as follows: 1) In degenerative spondylolisthetic L3 and L4 spines, facets were arranged in sagittal plane and sacrum was vertically arranged, but in L5 facets were coronally arranged and sacrum and in horizontal plane. 2) The lumbosacral angle was 149.9 and the average difference between anterior and posterior height of L5 vertebral body was 3mm. These results revealed that sacrum was more vertical and the 5th vertebral body was more rectangular, so lumbar lordosis was decreased as a whole. 3) The intercristal line was passed at IA or L4–5 interspace in 72.1% of cases(normal:95.8%), so L4 vertebra was placed in higher level. 4) In group A posterolateral fusion was done in 2 cases. Among group B and C, operation was done in 9 cases;4 cases with laminectomy and 5 cases with laminectomy and posterolateral fusion. Two years after surgery, slipping and instability were far advanced in 2 laminectomized cases for whom posterolateral fusion was followed later. 5) Unstable group of degenerative spondylolisthesis with severe neurological symptoms and claudication must be treated by decompressive laminectomy and posterolateral fusion, especially in patients under 60 years of age. 6) Young woman having flattened lumbar curve without slipping, narrowed disc space of L4–5, high-seated L4 and vertical sacrum is thought to be in prespondylolisthetic stage of degenerative origin; therefore, intensive back muscle exercise to prevent the slipping should be performed and periodic X-ray examinations should be followed up to detect the progress of the disease.