Whole blood filtration was performed in 33 patients with vascular disease (23 peripheral and 10 cerebral vascular disease) and in 15 controls using a polycarbonate membrane of 11 micron thickness with 5 micron pores. Blood filterability (index of red cell deformability) was reduced in patients with peripheral vascular disease and even more so in patients with cerebral vascular disorders. Blood glucose, systolic and diastolic blood pressure, among the risk factors, were significantly higher in patients with vascular disease than in controls but no correlation was between these factors and VRBC index (filtration rate/Hct).