In addition to haemoglobin concentration, haemoglobin oxygen affinity plays a major role in the oxygen releasing capacity of the blood. In this study we have measured oxygen affinity as P50 and calculated the oxygen releasing capacity of blood from 10 haemodialysis patients treated with erythropoietin (rHuEpo). The patients were examined with different assays before start of treatment, after 11 weeks, and after 27 weeks. During the first phase of treatment the oxygen releasing capacity improved because of an increase in the haemoglobin concentration and P50. During the second phase there was a further significant increase in haemoglobin concentration, but due to a decrease in the P50 value the oxygen releasing capacity remained unchanged. Despite an unchanged oxygen releasing capacity and total blood volume, the antihypertensive treatment had to be increased during that phase of treatment. An increase in whole-blood viscosity may explain the increased need of antihypertensive drugs. The increase in P50 during the first phase of rHuEpo treatment can probably be explained by decreased mean age of the erythrocyte population and implies that the beneficial effect is greater than could be concluded from the increase in haemoglobin concentration.