A 48-year-old man developed numbness in the left leg, which progressed gradually to paresis and urinary incontinence. Neurologic examination revealed a left Brown-Sequard syndrome with leg paresis, mild spasticity, reduced proprioception, and contralateral thermal and painful hypoesthesia below T6. MRI revealed a thoracic spinal cord herniation (SCH) (figure). Idiopathic SCH is relatively rare. Pathogenesis involves a dura mater defect (see video on the Neurology® Web site at Neurology.org); herniation develops over a progressive pressure gradient through the dural fissure. 2 Surgical reduction is typically performed if symptoms progress, but mild symptoms may be eligible for conservative treatment and monitoring. Surgical spinal reduction and dural repair usually reverses neurologic deficits.