Study of twenty skeletons and two hundred patients with degenerative spondylolisthesis established that it occurred four times more frequently in females, six to nine times more frequently at the interspace between the fourth and fifth lumbar vertebrae than at adjoining levels, three times more frequently in blacks than in whites, and four times more frequently the fifth lumbar vertebra was sacralized. It did not occur before the fifth decade or in conjunction with spina bifida or isthmic spondylolisthesis. The slipping occurs as a result of degenerative disease of the articular processes, but it never exceeds 30 per cent. When symptoms are severe and unrelieved by conservative treatment (10 per cent of our patients), decompression laminectomy and excision of the medial portion of the articular processes affords relief of pain. The predisposing factor is a straight, stable lumbosacral joint which puts abnormal stress on the intervertebral joint between the fourth and fifth lumbar vertebrae, leading to decompensation of disc and ligaments, hypermobility, and degeneration of the articular processes allowing forward slipping.