1. A clinical condition is described in which there are symptoms of compression of the caudal nerve roots on standing or walking, but not at rest. Seven cases are reported.
2. Myelography showed a block in the lumbar region in every case.
3. At operation narrowing of the spinal canal in part of its lumbar course was found.
4. The nature of the abnormality is discussed. It is suggested that the narrowing is due to encroachment on the spinal canal by the articular processes.