Degenerative spondylolisthesis occurs most frequently at the 4th lumbar vertebra in aging females. It does not occur before the fifth decade or in conjunction with neural arch defects. The amount of slipping does not exceed 30 per cent. The forward slipping occurs as a result of forward remolding of the z-a joints secondary to localized degenerative arthritis of these joints. The degenerative arthritis results from instability of L4 in spines which are unduly stable at the lumbosacral level. The degenerative changes and the forward slipping combine to produce localized spinal stenosis which may compress the nerve roots and cauda equina. The cardinal symptom is pain in the low back and/or lower extremity. It is characterized by remissions and is usually not incapacitating. Ten per cent of patients have sufficient pain and disability to require surgical decompression which is best accomplished by excision of the distal half of the laminae and spinous process of the slipped vertebra and the proximal half of the laminae and spinous process of the slipped vertebra. The medial half of each z-a joint is also excised to complete the decompression. If the decompression is adequate lasting relief of pain can be expected, but some low back symptoms due to instability may be noted postoperatively. These symptoms diminish with the passage of time.