<jats:p> <jats:italic>Background and Purpose</jats:italic> —Cerebral blood flow (CBF) is reduced after subarachnoid hemorrhage (SAH), and symptomatic vasospasm is a major cause of morbidity and mortality. Volume expansion has been reported to increase CBF after SAH, but CBF values in hypervolemic (HV) and normovolemic (NV) subjects have never been directly compared. </jats:p> <jats:p> <jats:italic>Methods</jats:italic> —On the day after aneurysm clipping, we randomly assigned 82 patients to receive HV or NV fluid management until SAH day 14. In addition to 80 mL/h of isotonic crystalloid, 250 mL of 5% albumin solution was given every 2 hours to maintain normal (NV group, n=41) or elevated (HV group, n=41) cardiac filling pressures. CBF ( <jats:sup>133</jats:sup> xenon clearance) was measured before randomization and approximately every 3 days thereafter (mean, 4.5 studies per patient). </jats:p> <jats:p> <jats:italic>Results</jats:italic> —HV patients received significantly more fluid and had higher pulmonary artery diastolic and central venous pressures than NV patients, but there was no effect on net fluid balance or on blood volume measured on the third postoperative day. There was no difference in mean global CBF during the treatment period between HV and NV patients ( <jats:italic>P</jats:italic> =0.55, random-effects model). Symptomatic vasospasm occurred in 20% of patients in each group and was associated with reduced minimum regional CBF values ( <jats:italic>P</jats:italic> =0.04). However, there was also no difference in minimum regional CBF between the 2 treatment groups. </jats:p> <jats:p> <jats:italic>Conclusions</jats:italic> —HV therapy resulted in increased cardiac filling pressures and fluid intake but did not increase CBF or blood volume compared with NV therapy. Although careful fluid management to avoid hypovolemia may reduce the risk of delayed cerebral ischemia after SAH, prophylactic HV therapy is unlikely to confer an additional benefit. </jats:p>