Diabetes mellitus markedly increases the risk of myocardial infarction, stoke, amputation which cause most morbidity and mortality. Elevated low shear blood viscosity with erythrocyte aggregation and elevated high shear blood viscosity with reduced erythrocyte deformability might be important and potentially treatable factors in the etiology or progression of diabetic macroangiopathy. The metabolic abnormalities caused by diabetes induce both rheological changes of blood and vascular dysfunction that predisposes this patient population to atherosclerosis. Lowered shear stress and vortices with oscillatory shear stress induce atherosclerosis. Abnormalities in platelet function and increased blood coagulability may exacerbate the progression of atherosclerosis and the consequences of plaque rupture or erosion in diabetes.