[en] Cerebral blood flow (CBF) reactivity to acetazolamide (ACZ) is useful to select patients with hemodynamic failure. However, it is still a matter of speculation that varying degree of regional CBF increases after ACZ administration represent the severity or stage of regional hemodynamic failure as assessed by positron emission tomography (PET). We studies to elucidate whether ACZ challenge ¹²³I-IMP brain single photon emission tomography (SPECT) can accurately grade the severity of regional hemodynamic failure. Eighteen patients (M: 16, F: 2, average age: 61 years) with unilateral occlusive disease of the internal carotid artery or the trunk of the middle cerebral artery (MCA). Patients underwent ¹²³I-IMP brain SPECT study with acetazolamide challenge and PET study was carried out within 2 weeks before and after SPECT study. Five healthy volunteers with a mean age of 48 years (range: 28-73yr, M: 3, F: 2) underwent PET studies to determine normal values. In SPECT study, an asymmetry index (Al)-the percentage of radioactivity of region of interest (ROI) in the occlusive cerebrovascular lesion to the contralateral homologous ROI-was used for numerical evaluation of relative ¹²³I-IMP distribution. In PET study, regional CBF, oxygen extraction fraction (OEF), cerebral metabolic rate of oxygen (CMRO₂) and cerebral blood volume (CBV) values were measured with ¹⁵O-labeled gas inhalation method and the values were used for comparison with Al (Al during acetazolamide challenge-Al of basal study) on the SPECT study. ROIs were classified by severity into three groups (normal, stage I and stage II). Mean values of Al in areas with normal, stage I and stage II hemodynamic failure were 6.25±7.77% (n=107), -10.38±10.41% (n=117) and 13.30±10.51% (n=140), respectively. Al significantly differed with each groups (p<0.05) Correlation between Al and CBF, OEF and CBV/CBF in hemisphere with occlusive cerebrovascular lesion was 0.20 (p<0.01), -0.28 (p<0.01), -0.28 (p<0.01), respectively. We concluded that ¹²³I-IMP brain SPECT with acetazolamide challenge could determine the severity and stage of regional hemodynamic failure as assessed by PET