Diabetes is associated with several types of rheological abnormalities: 1. macrorheological ones, consisting of increased whole blood viscosity and plasma viscosity; 2. microrheological ones, consisting of a change in erythrocyte deformability, an increase in erythrocyte aggregation and in their adhesiveness to endothelial cells. Microrheological abnormalities in the leukocytes and platelets are associated with these changes and may precede them. Certain of these abnormalities are particularly observed in diabetics with macro and/or microangiopathy and may contribute to a reduction in tissular oxygenation and to the vascular complications of diabetes. The precise mechanisms remain to be clarified as well as the part played by the hypertension often found in diabetes.