In children with renal anemia the concentration of DPG of red blood cells and the O2-affinity of hemoglobin (P50) were determined before and under treatment with EPO. Before treatment the concentration of DPG was 6.25 mmoles/l cells and the P50 28.9 Torr. A rise of the hematocrit to about 0.30 was accompanied by an increase of DPG to 7.95 mmoles/l cells and of P50 to 32.1 Torr, respectively. Thus improvement of renal anemia by stimulation of erythropoiesis by means of EPO was accompanied by a decreased O2-affinity of hemoglobin and therefore by an additionally improved O2-supply to the tissues.