A case of intense unilateral cerebral vasospasm with appropriate neurological deficit following rupture of a posterior communicating aneurysm was followed with serial angiography. Associated with the initially intense spasm was subsequent regional hyperperfusion with early venous filling which disappeared as signs of focal cerebral atrophy ensued. These angiographical findings can explain previous reported autopsy demonstrations of focal infarction in the distribution of aneurysm-bearing cerebral vessels. Laboratory cerebral blood flow studies in conjunction with experimental subarachnoid hemorrhage indicate that constriction of major cerebral arteries to less than one-half of their normal caliber is associated with reduction in regional cerebral blood flow to infarction levels.