To evaluate the efficacy of 133Xe SPECT and Diamox test, for selecting patients with hemodynamic compromise, tests using 133Xe inhalation method and single photon emission computed tomography (SPECT) with acetazolamide (Diamox) were performed in twenty-three patients with chronic cerebral ischemia, before and after extracranial-intracranial bypass surgery or carotid-endarterectomy. All patients complained of TIA, RIND, or minor completed stroke, and cerebral angiography of all patients demonstrated severe stenosis or occlusion in the ipsilateral internal carotid artery or middle cerebral artery. Cerebral blood flow (CBF) was also measured 15 minutes after intravenous administration of 10-12 mg/kg Diamox, which is known as a cerebral vasodilatory agent (Diamox test). Our results revealed that all patients could be divided into four types according to the findings of their resting rCBF and Diamox reactivity. The patients who had normal resting rCBF and normal Diamox reactivity (type 1) were considered to have well-developed collateral circulation and normal cerebral perfusion pressure (CPP) in spite of severe occlusive lesions in the carotid system. Moderate vasodilatation due to reduced CPP was considered to occur in patients who had normal resting rCBF and decreased Diamox reactivity (type 2). The resting rCBF remained unchanged, but Diamox reactivity improved to the normal range after surgery in the patients of type 2 (p less than 0.01), which indicated the improvement of CPP and the resolution of the autoregulatory vasodilatation. Maximum vasodilatation or dysautoregulation was considered to occur due to the inadequate collateral flow and the severely reduced CPP in patients whose findings revealed decrease in the resting rCBF and impaired Diamox reactivity (type 3).(ABSTRACT TRUNCATED AT 250 WORDS)