In two studies using the 133Xe clearance method the effect of chronic intravenous and oral administration of pentoxifylline on regional cerebral blood flow (rCBF) in patients with chronic cerebrovascular diseases was evaluated. In both studies rCBF increased significantly. The effect was more prominent in hypoemic than in normoemic tissue which led to the suggestion that not only viscosity but other local factors were also positively influenced by the drug.