A previous study showed that regional cerebral blood flow, measured following intra-arterial injection of 133Xenon, increased by at least 15% in one-third of all areas investigated in 25 patients with different types of dementia after a 2-weeks' period of intravenous treatment with 600 mg pentoxifylline per day. In the present studies, 10 patients were treated orally with 400 mg pentoxifylline 3-times daily for 4 weeks and 10 untreated patients served as controls. Regional cerebral blood flow was measured over both hemispheres using a new non-invasive technique by inhalation of 133Xenon gas. Bihemispheric cerebral blood flow increased by 15.9% in the pentoxifylline group and by 1.7% in the controls. Hypoemic areas, in particular, showed a distinctly higher increase (+20.1%) in blood flow after pentoxifylline compared to controls (+0.7%), and these hypoemic areas were apparently more influenced than hyperemic or normoemic regions. These findings were confirmed by a second controlled series. These data suggest an improvement of cerebral blood flow by pentoxifylline in a certain number of brain tissue areas if the drug is given over periods of at least 2 weeks in patients with cerebrovascular disease and other types of dementia.