Red cell aggregation is one of the part of the properties of blood which determine blood flow in the impaired microcirculation. Erythrocyte aggregation does influence not only the viscosity at low rates of shear but leads to a vicious circle which again impairs the blood flow in the microcirculation. In the other hand it seems obvious that red cell aggregation leads to an inhomogenous distribution of microcirculatory blood flow (maldistribution) thereby decreasing the oxygen transport tissue. In a great number of diseases red cell aggregation is increased and in some diseases this fact is responsible for clinical symptoms. This view is supported by the therapeutic benefit seen in patients with paraproteinaemia as well as in patients with vascular diseases treated with plasmapheresis or defibrinogenation, respectively. There is no doubt about, a reduction of red cell aggregation by desaggregating drugs to improve microcirculatory blood flow and do increase the oxygen supply to tissue. It is however hard to differentiate between the influence of red cell aggregation and/or plasma viscosity on the impaired microcirculation since both parameters are closely connected in most of these diseases.