The induced hemodilution is the most simple and efficient therapeutic measure of improving hemorheological parameters. Decrease of hematocrit, lowering of fibrinogen concentration and reduction of plasma viscosity are the mainly involved factors. They are to improve microcirculatory perfusion via amelioration of blood fluidity. The hematocrit value of 45% represents the critical borderline for the selection of the relevant procedure. Indications and limitations for the hypovolaemic (phlebotomy), iso- or normovolaemic and hypervolaemic dilution are reviewed with regard to the changes of the various hemorheological factors. Preference is given to the use of hydroxyethyl starch solutions.