Background and Purpose— It is still unclear that impaired cerebrovascular reactivity (CVR) to acetazolamide is comparable to elevated oxygen extraction fraction (OEF) on positron emission tomography (PET) in patients with occlusive carotid diseases. Therefore, in this study, the authors aimed to clarify whether OEF is elevated in all patients with reduced cerebral blood flow (CBF) and CVR (type 3) on single photon emission computed tomography (SPECT), and, if not, to specify the underlying pathophysiology of type 3 but normal OEF. Methods— This study included 46 patients who had decreased CBF and CVR on N-isopropyl-p-123I-iodoamphetamine SPECT in the ipsilateral middle cerebral artery area attributable to occlusive carotid diseases. Hemodynamic and metabolism parameters were determined in all patients by 15O-gas PET, and neuronal integrity was evaluated in 19 patients using 11C-flumazenil (FMZ) PET. Results— OEF was significantly elevated in 20 (43.5%) of 46 type 3 patients. Another 26 type 3 patients had...