 Cerebral vasospasm is a recognized and poorly understood complication for many patients who have aneurysmal subarachnoid hemorrhage. Constriction of the cerebral arterial vasculature occurs as free subarachnoid blood under high pressure comes into contact with the surfaces of vessels, particularly in the basal cisterns. However, the exact pathophysiology of vasospasm is unknown. Morbidity and mortality rates for vasospasm are high despite improvements in management. Excluding the initial hemorrhage, cerebral vasospasm is recognized as the main cause of substantial disability and death. Cerebral vasospasm kills 7 of patients, and causes severe deficit in another 7. Cerebral vasospasm almost never occurs before day 3, has maximal incidence at days 6-8, and rarely occurs after day 17. The disorder is clinically characterized by confusion or decreased consciousness with focal neurological deficit. 