1. Fifteen patients with spina bifida occulta are described in whom the fifth lumbar spinous process was pressing on the fibrous membrane that closes the spina bifida, or on the bony stumps of the defective lamina.

2. That this is a cause of low back pain, with or without pain referred to the lower limbs, is supported by the findings at operation and the results of treatment.

3. Eleven patients were treated by operation; in each the spinous process of the fifth lumbar vertebra pressed against the spina bifida, either directly or through the vestigial remnant of the first sacral spinous process.

4. In eight cases the fibrous membrane was adherent to the dura mater; in one case the nerve roots were adherent.

5. Excision of the fifth lumbar spinous process and the membrane across the breach of the spina bifida was an effective method of treatment.