• A 20-year-old man suffered head, chest, and abdominal trauma in an auto accident resulting in a traumatic dissecting aneurysm of the thoracic aorta. Hypotension developed. The aneurysm was resected and replaced with a prosthetic graft. Postoperatively, the patient was found to be paraplegic below T-9, areflexic and anesthetic to pain and temperature, with preservation of vibration and position senses. In the ensuing nine months, the patient regained considerable sensory function in his lower extremities and had severe constant hyperhydrosis below the T-9 dermatome. The exaggerated sweating was unaffected by temperature change and anxiety. It was diminished by methantheline bromide treatment but never abolished. The spinal cord lesion is postulated to be anterior horn cell loss, with preservation of interneurons and intermediolateral gray columns. Disinhibition of sympathetic circuits or sprouting of axons are proposed mechanisms.