Recently we had experienced a case of posterior migration of herniated lumbar nucleus pulposus over the cauda equina, simulating a cord tumor clinically and radiologically. But we could hardly find any previous case report on it. A 35 years old man was admitted to our ward due to paraparesis on July 17, 1975. He had a history of back sprain five years ago with intermittently appearing mild lumbago. Two weeks prior to admission, there appeared severe left leg pain and soon resulted in paraparesis. Preoperative positive neurologic findings were paraparesis with left side foot drop, hypesthesia on the left L4, 5 and S1 dermatome and bilaterally decreased ankle and knee jerks. Myelogram revealed complete block at L2-3 with suspicious indentation filling defect at left L-5 interspace. C.S.F. protein value marked 738mg%. On operation, the dural sac was compressed by a posteriorly overriding mass on it at the L2-3 level. The mass was traced to its origin from the left side of the L2-3 interspace and into the ruptured posterior longitudinal ligament with a narrow pedicle. L4-5 interspace revealed a typically protruding type disc lesion. These were composed of degenerated nucleus pulposus on histological examination.