✓ A detailed study of posttraumatic syringomyelia is reported. The interior of the syrinx was outlined by positive contrast and gas myelography. The contrast material entered the syrinx via a communication between the cavity and the subarachnoid space at the site of spinal cord injury. The syrinx also communicated with the fourth ventricle. It is postulated that posttraumatic syringomyelia results from the dissection of cystic remnants of hematomyelia known to be present at the site of serious spinal cord injury. Dissection occurs when pressure within the cyst is increased by elongation of the spinal cord during neck movements, principally flexion. Posttraumatic syringomyelia should be treated by a surgical procedure, which allows permanent drainage of the syrinx into the subarachnoid space.