The behaviour of regional cerebral blood flow was studied in 90 patients with vascular type dementia. Patients were divided at random into three groups of 30 and treated 3-times daily over a period of 8 weeks with either 400 mg pentoxifylline ('Trental' 400) or 2 mg co-dergocrine mesylate, or remained untreated (control group). Measurements of regional cerebral blood flow were made before and after 4 and 8 weeks of the study using an atraumatic inhalative 133Xenon clearance technique and assessments made in 16 regions of interest per hemisphere (grey matter perfusion). A statistically significant increase over baseline in mean regional cerebral blood flow was found in patients on pentoxifylline medication at Weeks 4 and 8. At Week 8, the change from baseline was +16.4% in the pentoxifylline group whereas the respective values for the other two groups were +0.4% for co-dergocrine mesylate, -2.4% for the controls. Hypoemic regions showed the most pronounced regional cerebral blood flow changes with pentoxifylline (+40%), the corresponding values for the co-dergocrine mesylate and control group being +10.8% and +0.4%, respectively.