Recombinant human erythropoietin (rhu EPO) is the choice treatment of dialytic anemia; however, this therapy has side effects due to the increased number of blood components involved. It seemed to us worth assessing, by hemorheological study, the impact of such a treatment on blood flow properties, already impaired in this type of patients. This study was designed to measure the evolution of hemorheological parameters in 16 hemodialysed patients before and after 2.3 and 6 months of treatment with rhu EPO. Hemorheological work-ups included: erythrocyte filtration with a hemorheometer; blood and plasma viscosities (LS30), ATP and 2.3 DPG, RBC aggregation (Sefam erythroaggregameter), RBC morphology under a scanning electron microscope; blood counts and full biochemical work-ups were performed to explore renal function. The results showed, besides a significant increase in hemoglobin: normalized rigidity index, reflecting the better deformability of erythrocytes; a moderate increase in blood viscosity with uncorrected hematocrit, becoming significant after 6 months of treatment. This increase however did not reach the values that could be expected with the increased hematocrit (it was probably balanced by improved erythrocyte deformability, which is confirmed by the fact that with corrected hematocrit, blood viscosity decreases during treatment). Studying erythrocyte aggregation in hemodialysed patients reveals, in the absence of any treatment, a decrease in aggregation time and a higher dissociation threshold, which reflects a tendency to erythrocyte hyperaggregation enhanced by erythropoietin.(ABSTRACT TRUNCATED AT 250 WORDS)