A haemorheological analysis with measured blood filterability was carried out on 179 patients with cerebrovascular accidents. Reduced blood filterability appeared to be related to the clinical condition: time variation (two phase course), influence of complications and of fatal prognosis. The underlying mechanism of the haemorheological disorders involved three groups of factors: quantitative and qualitative abnormalities of plasma proteins, red cells disturbances and white cells activation, leading to both local and general hyperviscosity, producing a prothrombotic state and a defective microcirculation. The haemorheological treatments are of value to the patients although the relationship between hemodilution and the filterability remains to be exactly defined, as well as a better understanding of the action of the drugs.