Transcranial Doppler ultrasound was used to determine the natural history of cerebral arterial vascular spasm following severe head injury. Doppler Flow Velocity (DFV) was measured in the anterior cerebral, middle cerebral and terminal carotid arteries. High DFVs consistent with vasospasm were identified in 68% of the 25 patients studied. They came on from between 12 h and 4 days and lasted from 12 h to 14 days. No relationship could be determined between presumed vasospasm and blood pressure, intracranial pressure or outcome. Rises in the maximum DFV value (to above 100 cm s-1) were associated with neurological deterioration in 12 instances (in 11 of these the DFV remained elevated for more than 12 h). Deterioration occurred in three instances without change in DFV. DFV rose to above 100 cm s-1 on 10 occasions without neurological change (but remained elevated for more than 12 h in only four of these). Vasospasm has been found to be more common following head injury and may be a more significant cause of deterioration than previously thought.