To evaluate the comparability of hemorheologic parameters in arterial and venous blood, we measured hematocrit, whole blood viscosity, plasma viscosity, erythrocyte deformability, erythrocyte aggregation, and erythrocyte indices in both arterial and venous blood from 20 consecutive patients scheduled for coronary artery surgery and/or valve replacement surgery. Hematocrit, whole blood viscosity at three shear rates (0.05 s-1, 0.5 s-1, and 70 s-1), plasma viscosity, and erythrocyte aggregation factor were statistically significantly higher in venous blood than in arterial blood. The differences may be explained by the difference in hematocrit. Erythrocyte deformability did not differ significantly. With the availability of more precise rheological measurement techniques, differences such as those encountered in this study may be of importance in clinical studies. It is concluded that arterial and venous blood samples are not entirely rheologically comparable.