1. The forward slip of the vertebral body and the defect in the pars interarticularis in spondylolisthesis represent only a part of the anatomical alteration present in such cases.

2. The arch of the displaced segment tends to provide support for the spine and to prevent further slippage. Removal of the arch is advisable, but only if replaced by a bone graft to give adequate support.

3. Pain in spondylolisthesis is the result of more than one etiological factor. We believe it to be due to impingement of malaligned bony elements and to abnormal stress on fibrous or ligamentous structures.

4. Extradural pressure from prolapse of an intervertebral disc occurs occasionally. Pressure from hypertrophied fibrocartilaginous tissue in the pars interarticularis defect has not been noted by us. Disc degeneration per se cannot cause pain.

5. Spine fusion by the technique described can be obtained in 85 per cent. of patients primarily and in 90 per cent. following repair of pseudarthroses. Relief of symptoms resulted in 89 per cent. of the patients with solid fusion.

6. Once solid fusion has been obtained, bony support of the weakened area is not only re-established, but it is permanent.