Whole blood, plasma, and serum viscosity together with red cell deformability were measured before and during treatment of renal anaemia with recombinant human erythropoietin (EPO). Whole blood viscosity (WBV) progressively increased during the first 4 months of treatment in association with the rise in haemoglobin concentration. When the WBV was corrected to a standard haemoglobin concentration no change in blood viscosity was observed, neither was there any alteration in a derived index of red cell deformability, or in the plasma and serum viscosities. In addition, a direct measurement of red cell deformability using a filtration technique before EPO therapy was similar to that obtained in 30 healthy volunteers. There was no significant change in this parameter over the first 9 months of treatment. The rheological changes which occur with correction of anaemia with EPO can be explained solely by the increase in circulating haemoglobin mass rather than to any change in the properties of the plasma or red cells themselves.