Blood flow is a complex process combining fluid shearing in both plasma and the interior of red blood cells with elastic deformation of blood's solid elements. The red cell membrane is the major solid in blood, but platelets and white cells contribute solid behavior as well. Changes in blood's flow properties are often hidden by blood's ability to change its pattern of response. Capillary viscometry can be used to examine serum, plasma, and hemoglobin solutions directly, but rotational viscometry, where regularity of fluid shearing can be controlled to a narrow shear rate range, is required to study blood flow effectively. The most striking diabetic blood flow abnormality is best seen in time-based rotational viscometer studies that demonstrate the pattern of development of shear stress as flow becomes established. In such studies blood demonstrates both viscoelastic and thixotropic behavior; in diabetes blood's thixotropy is substantially increased. The diabetic pattern appears to be produced by a combination of reduced erythrocyte deformability and increased erythrocyte aggregation due to plasma protein changes. The plasma protein changes observed in diabetes are linked to the development of glucose intolerance but they are not specific to diabetes. The combined increase in aggregation and resistance to deformation of red blood cells produces blood flow abnormalities that can be detected primarily at low shear rate. Plasma protein changes and alteration in blood viscosity are absent from children with diabetes, while such changes tend to be associated with diabetic microangiopathy in adults. In some in vivo conditions, the effects on blood flow that can be linked to erythrocyte deformability are magnified out of proportion to measurements made in vitro. Vessel wall injury may be produced by the adult diabetic hemorheologic changes, contributing to the development of both diabetic microangiopathy and atherosclerosis.