The blood of patients with chronic global respiratory failure and polycythemia exhibits higher viscosity than that of normal subjects. Plasma changes have been excluded as causal factors. The viscosity of whole blood, plasma and blood after correction of hematocrit (Ht; 45%) with autologous plasma, has been determined. The results indicate that in such patients the increase in Ht is not the most important factor affecting the rheological properties (increased viscosity) of blood. A high Ht value may be only pat of the mechanism that increases the blood viscosity of patients with chronic obstructive pulmonary disease. There is evidence that other factors are responsible for increased blood viscosity in chronic respiratory failure.