Regional blood flow and oxygen metabolism were determined by positron emission tomography using the steady-state technique with 150, in 2 patients with posthypoxic ischemic leukoencephalopathy, in 6 patients with leuko-araiosis and dementia, and in 6 patients with leuko-araiosis without dementia. Comparison between the three groups allowed us to conclude that leuko-araiosis is associated with a lowered cerebral blood flow, which is more pronounced and widespread in demented patients and more restricted to the white matter in nondemented ones.