Superficial temporal artery-middle cerebral artery (STA-MCA) bypass is widely accepted as an effective treatment for ischemic cerebrovascular disease in terms of improvement of cerebral blood flow in the ischemic region. However, the indications for and timing of STA-MCA bypass in the acute stage of ischemic stroke have not been established. Isovolemic hemodilution has been shown to temporarily raise collateral perfusion in acutely ischemic brain regions distal to occluded cerebral arteries. The authors performed isovolemic hemodilution in 26 cases of cerebral ischemia within 72 hours of the onset of symptoms. This resulted in a 30% increase in regional cerebral blood flow (rCBF) in the area supplied by the affected MCA, leading to rapid neurological improvement in 18 patients. Nine of these 18 were randomly assigned to the early bypass group, in which STAMCA bypass was performed within 2 days of hemodilution. In the delayed bypass group, which included the remaining nine patients, bypass was performed more than 30 days after hemodilution. In the early group, rCBF in the affected MCA region increased by 18% and the neurological score (NS) improved by 10% within 1 month postoperatively. In the delayed group, follow-up examination 1 to 2 months after hemodilution, rCBF in the affected MCA region and NS had returned nearly to pretreatment levels. Moreover, in this group STA-MCA bypass improved neither rCBF nor neurological status. It can be concluded that isovolemic hemodilution temporarily increases rCBF in the ischemic territory and enhances the potential reversibility of neuronal damage in patients with acute ischemic stroke. Neuronal damage may be reversed and rCBF may be significantly improved if blood flow is re-established by surgical means (STA-MCA bypass) within hours or days following hemodilution.