Posttraumatic cerebral arterial spasm (vasospasm) has been demonstrated in the past by angiography, and recently by transcranial Doppler ultrasonography. Posttraumatic vasospasm is a delayed complication that involves the large basal intracranial arteries (e.g., internal carotid, middle cerebral, basilar) and occurs in 25-40% of head trauma patient. The time course of posttraumatic vasospasm resembles that of vasospasm associated with aneurysmal subarachnoid hemorrhage with onset occurring 2 or more days after injury. A study of the relationship of admission CT scan findings to the incidence of vasospasm suggests that intradural bleeding, which extends into the CSF (subarachnoid, intraventricular, and subdural hemorrhage), plays a role in the pathogenesis of posttraumatic arterial spasm. The preliminary results of a large prospective study of head trauma patients suggest that vasospasm may be an important determinant of outcome from severe head injury.