Intradural extension of a herniated intervertebral disk, an unusual complication of a common disease, may mimic an intradural tumor on MR. A case of a pathologically proved subdural disk fragment is presented; MR findings that suggested the correct diagnosis were: proximity of intervertebral disk disease; whorl-like mixed intensity on T2-weighted images; poor visualization of the mass on unenhanced T1-weighted images; and marked ring enhancement following administration of gadolinium.