Many neuropathological studies have suggested that periventricular hyper intensity (PVH)lesions on MRI T2 weighted image reflect ischemia. Intimate relation between PVH and dementia was also suggested in some clinical studies. This study was planned to investigate the relation between PVH, ischemia and dementia using positron emission tomography (PET) Thirty-three patients with subcortical multiple cerebral infarct and 7 normal subjects were studied. Patients were classified under prevalence of PVH on MRI T2 weighted image ; PVH(+) group : 15 patients with severe PVH and PVH(-) group : 13 patients without PVH. All patients were examined neuropsychologically for evaluating their intellectual function. Cerebral blood flow (CBF), cerebral metabolic rate of oxygen (CMRO_2), cerebral oxygen extraction fraction and cerebral blood volume (CBV) were measured by PET using C^<15>O_2, ^<15>O_2,^<11>CO inhalation technique. In none dementia patients with PVH, significant decrease in CBF and significant increase in OEF were recognized in the cingulate gyrus and PVH lesions compared with normal group. However, there was no significant difference in CMRO_2. In PVH(+) group with dementia, significant decrease in CBF, significant increase in OEF and significant decrease in CMRO_2 were recognized in PVH lesion and all cortical regions compared with normal control. CBF/CBV ratio in the PVH lesion and some cortical regions in PVH(+) group decreased significantly compared with PVH(-) group. These results indicate that PVH in patients with multiple cerebral infarction relates to hypoperfusion, which might be caused by cerebral arteriosclerosis. We suggest that continuity of hypoperfusion induces irreversible disturbance of neuronal function, which followed by dementia.