This is a report of a patient who developed sharp intercostal pain and Brown-Séquard syndrome. Displacement of the spinal cord toward an extradural mass was noted at the T4-5 vertebral level on iophendylate myelography and metrizamide computerized tomography myelography. Multiple meningeal diverticular lesions of congenital origin were also found. Surgical correction of the spinal cord, which had herniated into a laterally located extradural arachnoid cyst and become incarcerated, resulted in a complete neurological recovery.