&NA;Red blood cell mass and total blood volume were significantly decreased in 15 nonselected patients with subarachnoid hemorrhage. Possible causes of the contracted blood volume include bedrest, supine diuresis, pooling in the peripheral vascular beds, negative nitrogen balance, decreased erythropoiesis, and iatrogenic blood loss. The data support the use of intravascular volume expansion with red blood cells and colloid in the prevention and treatment of ischemic complications associated with ruptured intracranial aneurysms.