Traumatic dissections of the extracranial internal carotid artery (ICA) in 18 patients aged 19 to 55 years were studied. All had suffered blunt head or neck injury of marked or moderate severity; motor-vehicle accidents were the leading cause of the injury. Delayed focal cerebral ischemic symptoms were the most common presenting symptoms. Less commonly noted was focal unilateral headache associated with oculosympathetic paresis or bruit. Following a head injury, the abrupt onset of focal cerebral symptoms after a lucid interval should raise the suspicion of arterial injury, particularly when computerized tomography fails to show abnormalities that would explain the evolving neurological deficits on the basis of direct trauma to the brain. Unilateral headaches, oculosympathetic palsy, and bruits also help in establishing the diagnosis. Focal cerebral ischemic symptoms may develop months or years after the initial trauma. These delayed symptoms are caused by embolization from a thrombus within a residual dissecting aneurysm. Common angiographic findings, in decreasing order of frequency, are: aneurysm, stenosis of the lumen, occlusion, intimal flap, distal branch occlusion (embolization), and slow ICA-to-middle cerebral artery flow. Although two patients died as the result of massive cerebral infarction and edema and some were left with severe neurological deficits, most made a good recovery. Residual dissecting aneurysms and occlusion seem to occur more frequently with traumatic dissections than with spontaneous dissections of the extracranial ICA.