Forty-five patients with subarachnoid hemorrhage due to verified intracranial aneurysms were studied prospectively to determine whether delaying operations in those patients with abnormal cerebral perfusion, assessed by radionuclide dynamic scanning, would lower case management mortality. Twenty-nine patients had intracranial operations when their radionuclide dynamic scans demonstrated normal perfusion. The one death in this group occurred in a patient who suffered a massive hemorrhage during operation as the bone flap was elevated. There were no instances of delayed spasm after operation. Of the 16 remaining patients who were treated without operation, 5 died. Only 1 of these deaths resulted from recurrent hemorrhage. This patient had normal cerebral perfusion but was not operated upon because of severe associated medical problems. Cerebral infarction occurred in 10 of the 16 unoperated patients, all of whom had persistently decreased cerebral perfusion. Four of these patients died of their infarctions or related causes. Although a trend related vessel caliber as seen on arteriograms and perfusion delay seen on dynamic scanning, individual exceptions were numerous. This study shows that radionuclide scanning, a safe procedure, is useful for identifying two types of patients: those with normal perfusion prone to rebleeding in whom an early operation is safe and those in whom perfusion is decreased. The latter group is prone to cerebral infarction, but rebleeding from the aneurysm is unlikely. Operation should be delayed in these patients until cerebral perfusion returns to normal or for several weeks if cerebral perfusion remains diminished.