Seventeen haemodialysis patients with renal anaemia were treated with recombinant human erythropoietin (rhEPO) and observed for 30 weeks. The viscosity of whole blood and plasma, the erythrocyte aggregation tendency, and the erythrocyte deformability, measured as fluidity, were analysed every second week. All patients responded with increasing haematocrit and whole-blood viscosity. The plasma viscosity and the erythrocyte aggregation tendency were already increased before the start of treatment, and remained unchanged during treatment. The basal erythrocyte fluidity tended to be impaired, although not significantly so. During treatment, significant impairment of fluidity was observed at the beginning of the treatment period. After 24 weeks the fluidity started to increase, and it later reached values observed before the start of treatment. Hence, the quality of the erythrocytes formed during the corrective phase of rhEPO treatment differs in some respects from that of cells formed at a normal production rate. The impaired fluidity might have important implications for the flow resistance in small vessels, and contribute to the development or aggravation of hypertension that is often seen during rhEPO treatment.