The effect of pentoxifylline on cerebral blood flow (CBF) was studied with the intravenous 133Xe clearance technique in eleven patients with chronic cerebrovascular disease. Pentoxifylline was administered orally at a dose of 1200 mg/day over a period of 30 days (eight patients) or by intravenous infusion of 100 ml saline containing 400 mg of the drug in 1 hour (three patients). CBF was measured before and after pharmacological treatment. CBF was found to be significantly increased by both acute and chronic administration of pentoxifylline.