For about every 25 operations performed for protruded lumbar intervertebral disk at the Mayo Clinic, one operation is performed for spinal cord tumor that is so situated that, at some time during its course, it could simulate a protruded lumbar intervertebral disk. One of our cases illustrates this statement especially well. A man, 41, a white collar worker, came to the Mayo Clinic in 1940 complaining of low back pain, with extension along the left sciatic nerve. His symptoms had been present intermittently for the previous three years. Examination revealed slight limitation of forward bending, Laségue's sign of sciatica on the left, diminution of the quadriceps reflex in the left leg, and slight weakness of the left leg. Simple roentgenographic examination of the thoracic and lumbosacral portion of the spinal column showed nothing abnormal. A provisional diagnosis of protruded intervertebral lumbar disk was made and intraspinal investigation was recommended. The