Rheological parameters were measured in two cross-sectional studies involving children and adults suffering from diabetes mellitus type I and adults with diabetes mellitus type II. Early changes are increases in spontaneous thrombocyte aggregation, erythrocyte rigidity, and plasma viscosity. Typ II diabetics with numerous concomitant diseases, especially concerning the vascular system, present considerable restrictions in almost all rheological parameters. It is known that insufficient blood flow increases the incidence of arterial occlusive disease. Also in these investigations the rheological parameters increased with the degree of atherosclerosis. Moreover, the quality of blood sugar control influences the fludidity of blood. A therapeutic adjustment of rheological parameters improves clinical symptoms of circulatory disorders. Therefore, it seems to be useful to determine rheological parameters for the early diagnosis and efficient therapy of vascular diseases.