The aggregation of erythrocytes plays an essential part in vascular diseases involving both the veins and the arteries. In the veins, where the lowest blood flow rates are observed, the conditions of flow foster the aggregation of the erythrocytes, which accounts for blood hyperviscosity that may reinforce the effects of the stasis. In arterial diseases, hyperaggregation of the erythrocytes resulting from risk factors such as smoking, high BP, diabetes, disorders of lipemia, etc. contributes to the formation of stenosis, which has severe consequences for the patient. The prospects of the treatment aimed at correcting the hemorheological disorders caused by hyperaggregation of the platelets are ruled by Poiseuille's formula and based on hemodilution processes and the use of agents lowering fibrinogenemia or molecules improving the forming properties of the red blood cells.