We report one case of spontaneous thoracic spinal cord herniation presenting with a progressive spastic paraparesis for 4 years in a 55 years old man. From preoperative MRI, showing a ventrally displaced atrophic spinal cord at T2-T3 level, a dorsal intradural arachnoid cyst was suspected. At operation, after a 3 level laminectomy, no arachnoid cyst was found and spinal cord herniation into a meningeal diverticulum was confirmed. The herniated myelon was replaced intradurally and the lumen of the diverticulum was filled with Teflon settled with fibrin glue to prevent recurrence. Postoperatively some neurological recovery was achieved. The literature was reviewed, regarding clinical and epidemiological features, proposed pathophysiological mechanisms, treatment options and outcome. Only 32 surgically proved cases of thoracic spinal cord herniation with no past history of spine trauma, injury or surgery were found.