8 patients of delayed CO sequelae were evaluated using Brain CT and Tc-99m-HMPAO SPECT. The results were as follows; 1) CT findings of delayed CO sequelae were bilateral low density lesion in globus pallidus (1 pt.), diffuse low density in white matter with bilateral low density in white matter (1 pt.), diffuse low density in white matter with bilateral low density in globus pallidus (1 pt.), diffuse low density in white matter with cortical atrophy (1 pt.), bilateral low density in globus pallidus and diffuse low density in white matter with cortical atrophy (1 pt.) and normal in 3 pts. 2) Tc-99m-HMPAO Brain SPECT findings of delayed CO sequelae were decreased regiona1 cerebral b1ood flow (rCBF) in frontal (1 among 8 pts.), frontal and basal ganglia (3 among 8 pts.), and diffuse patch decreased rCBF pattern (4 among 8 pts.) 3) Tc-99m-HMPAO Brain SPECT study was well correlated with neurologic symptoms and signs in delayed CO sequelae. Our results may suggest that reduced cerebral blood flow contributes to the development of delayed CO sequelae.