The relationship among ventricular size on computed tomography (CT), the clinical status of the patient, and cerebral blood flow alterations in subarachnoid hemorrhage is examined. Fifty patients with subarachnoid hemorrhage underwent a total of 71 cerebral blood flow measurements and 115 CT scans. Flow was measured noninvasively using 133Xe inhalation. It is demonstrated that increasing ventricular size is accompanied by clinical deterioration and also by a reduced flow. The effects of the various lesions (as demonstrated by CT) on cerebral perfusion are examined. Of the 23 patients scanned 5 days or less after hemorrhage, 12 had subarachnoid blood visible on the scan. These 12 had flows that averaged 20% lower than the other 11 when examined 6 to 17 days after the hemorrhage. Ventricular enlargement is more prevalent in the first week after the hemorrhage than in the second. Also, blood flow is maximally reduced at the end of the first week following hemorrhage.