Blood, plasma and serum viscosity, packed red cell volume, and plasma fibrinogen concentration was measured in a group of 13 subjects, suffering from peripheral obliterative arterial disease of the lower limbs (stage III or IV), awaiting surgery, and in 9 control subjects, none of whom showed signs of circulatory disease of the lower limbs either by clinical examination or by instrumental measurements. The blood samples were taken from the: (a) antecubital vein; (b) femoral vein; (c) femoral artery. In the patients venous blood viscosity was significantly higher than arterial blood viscosity. No significant differences have been found between the femoral and the antecubital vein, but in the femoral vein blood viscosity was slightly higher. Packed red cell volume and plasma viscosity were slightly but significantly higher in venous blood, while no difference was seen in serum viscosity and in plasma fibrinogen concentration. In the control subjects blood viscosity values were decidedly lower than in patients and no statistically significant differences were to be found between artery and vein, even if viscosity values were lower in artery. These results support the hypothesis that an interrelation exists between hyperivscosity syndrome and vascular ischaemia-inducing diseases.