Herniated lumbar discs may infrequently extrude through or around the posterior longitudinal ligament and migrate within the epidural space either as a contiguous mass with or as a free fragment of the parent nucleus pulposus. Recognition of this entity may obviate the use of chymopapain or may otherwise alter the surgeon's approach to the disc herniation. The CT scans of 40 surgically confirmed cases of lumbar disc extrusion were reviewed. In 36 cases CT identified the extruded disc material as a lobulated soft tissue mass of similar attenuation to the disc space, migrating either superiorly or inferiorly from the parent disc interspace. In 34 cases the extrusion was 6 mm or more from the center of the parent disc. Calcification of the disc extrusion occurred in five cases. In six cases an extruded free fragment of disc material was associated with a normal-appearing parent disc interspace at CT. In one such case gas bubbles occurred within a degenerating extruded fragment which had eroded through the dura. The differential diagnosis of lumbar disc extrusion is also discussed.