The anomaly of the viscosity of human blood is more pronounced in diabetics. This is caused by an increase in plasma viscosity, a more pronounced red-cell aggregation, and a reduction of individual cell deformability. The changes in viscosity and in red-cell aggregation both are the consequence of abnormal plasma proteins, the incidence of which is largely independent of the onset and duration of disease, and actual metabolic state. The presence of complicating infectious diseases further aggravates the pathologic red-cell aggregation. The decreased red-cell deformability is largely independent on onset, duration, and complications but depends critically in the incident metabolic control of the diabetics. The possible role of hemorrheologic factors in the development of microangiopathy is discussed.