Cerebral blood flow (CBF) was measured during the 1st week of subarachnoid hemorrhage in 46 patients who were in a good clinical grade and had a proven ruptured intracranial aneurysm. The mean initial CBF in patients who developed cerebral ischemia was 42 ml/min-1/100 gm brain-1, which was significantly lower than in patients who did not develop cerebral ischemia (49 ml/min-1/100 gm brain-1). This reduced CBF was not secondary to raised intracranial pressure or angiographic spasm. Patients with a reduced CBF (less than 50 ml/min-1/100 gm brain-1) and diffuse subarachnoid blood on computerized tomography had a very high incidence (78%) of cerebral ischemia, despite a good clinical grade at the time of measurement. Serial CBF measurements are of value in monitoring the evolution of cerebral vasospasm.