Although there is considerable literature concerning ossification of the posterior longitudinal ligament or the ligamentous flava (OPLL or OYL) in the cervical and thoracic spine, there are only a few references about OPLL or OYL in the lumbar spine. The authors have described lumbar spinal stenosis due to OPLL or OYL as hyperostotic lumbar spinal stenosis, and analyzed 12 surgically documented cases with this condition. The symptoms and signs of hyperostotic lumbar spinal stenosis are the same as those seen in degenerative lumbar spinal stenosis, but the degree of paraparesis is much more severe in hyperostotic lumbar spinal stenosis. Computed tomography scan imaging clearly demonstrates OPLL or OYL in the lumbar spine, although some lesions can be seen on the lateral view of a plain roentgenogram. The results of 12 surgical cases suggest that decompression laminectomy produces relief of symptoms. An analysis of 2,403 plain lumbar roentgenograms showed an incidence of 8.4% OYL in the lumbar spine, with frequent involvement of the upper and middle lumbar spine. A classification system of OYL in the lumbar spine has been developed. The entire spine should be examined before surgery on a patient with hyperostotic lumbar spinal stenosis because of a tendency to ossify spinal ligaments at other levels.