Tethered cord syndrome (TCS) usually involves tethering of the lower cord at the conus medullaris from dural abnormalities, but may occur after spinal cord herniation. A tethered thoracic spinal cord is rare. We present an unusual case of a 30-year-old woman with a history of myelopathy presumed to be secondary to T6 cord compression resulting from T6-T8 arachnoid cyst. She continued to deteriorate after partial excision of the cyst. Repeat magnetic resonance imaging suggested recurrence of the presumed arachnoid cyst with cord compression and showed tethering at T6-T8. Surgical exploration revealed myelocele with cord herniation through the anterior thoracic dura. Pathologic diagnosis showed neural tissue with gliosis. After physical therapy treatments, the patient had increased lower extremity strength, ambulated with a cane, and regained some bladder control. Progressive myelopathy may represent tethering of the cord resulting from cord herniation. Early recognition of TCS, even in patients with minimal neurologic deficits, could prevent progressive disability.