Leukoaraiosis, found on computed tomographic scans of the brain, is suspected to be of ischemic origin and is frequently associated with progressive decline of cognitive functions in elderly persons. Some of them also develop late-onset epilepsy. The present positron emission tomographic study investigates if these seizures in mentally nonaffected patients with leukoaraiosis are related to an underlying ischemic process. Patients with leukoaraiosis and late-onset seizures have a more important decline of regional blood flow and oxygen consumption in the cortical areas, compared to normal age-matched controls, to patients with cryptogenic symptomatic late-onset epilepsy and to patients with a similar degree of leukoaraiosis but without epilepsy. The present study suggests that both the late-onset seizures and the leukoaraiosis in patients with, at that time, no obvious mental deterioration are premonitory signs of an encephalopathy of unknown origin, possibly leading to cognitive decline.