Cerebral vasospasm is the classic cause of delayed neurological deterioration after aneurysmal subarachnoid hemorrhage, leading to cerebral ischemia and infarction, and thus to poor outcome and occasionally death. Advances in diagnosis and treatment-principally the use of nimodipine, intensive care management, hemodynamic manipulations and endovascular neuroradiology procedures-have improved the prospects for these patients, but outcomes remain disappointing.