The radiological and myelographic appearances of 14 patients with severe lumbosacral spondylolisthesis have been reviewed in relation to compression of the cauda equina and their clinical symptomatology. Patients could be separated into two groups as defined by the position of the L5 neural canal. In Group 1, the arch was displaced downwards and the cauda equina compression was due to pressure from the L5/S1 disc and was related to the degree of listhesis. L5 root sheath abnormalities predominated in this group. In Group 2 the neural arch maintained a more normal position and the compression, which was unrelated to the degree of slip, occurred between the arch and the posterior aspect of the sacrum. S1 root lesions occurred in these patients due to pressure from the arch. Clinical symptoms and signs were also more prominent in this group.