Delayed hypoxic encephalopathy is one of the most important neurologic sequelae in carbon monoxide (CO) poisoning. Regional cerebral blood flow (CBF) of cerebral cortices were measured in five patients with delayed CO encephalo'pathy confirmed by clinical features and magnetic resonance imaging (MRI) and five normal persons using "'Tc-HMPAO SPECT (technetium 99m hexamethylpropylene amine oxide single photon emission computed tomography). For the quantitative analysis, six pairs of region of interest in cerebral cortices and cerebellar hemispheres were determined. Among five regions to cerebellar ratios, those of superior and inferior frontal, parietal, and temporal cortices were significantly reduced (p<005). The degree of reduction in CBF was marked especially in frontal and temporal cortices and similar in both hemispheres. Early scan failed to predict delayed neurologic sequelae in one patient, but follow-up scan was well correlated with clinical improvement in another patient. Diffuse reduction of CBF in cerebral cortices was different from the anatomical MR findings. We think that reduced CBF in cerebral cortices is associated with clinical features of delayed carbon monoxide encephalopathy. Transient vascular mechan' ism or re duced cortical energy metabolism might be the cause of reduced CBF in cerebral cortices.