We used the isotope dilution technique to monitor circulating blood volume (CBV) in three patients with ruptured cerebral aneurysms who developed pre- or postoperative ischemic symptoms that responded well to intravascular volume expansion therapy with blood transfusion and plasma expanders. In the first and second cases, predeterioration CBVs were obtained. Both of these patients showed hypovolemia and a decreased red blood cell volume at the time of neurological deterioration. A predeterioration CBV was not available for the third patient for comparison, but his red cell volume was also markedly decreased. Postrecovery CBVs were obtained in the second and third cases. Our data suggested that a depleted red blood cell volume was more responsible for neurological deterioration than was a lowered plasma volume. To prevent the occurrence of hypovolemia and anemia in aneurysm patients, we should monitor CBV not only at the time of neurological deterioration, but also at the time of admission and during the immediate postoperative period.