Recombinant human erythropoietin (epoetin) is widely used for the treatment of renal anemia. The aim of our study was to determine the influence of epoetin on erythrocyte metabolism. Thirty-six hemodialysis patients (22 men, 14 female), aged from 17 to 64 years (mean age 43) and 30 healthy volunteers (12 men, 18 female), aged from 25 to 65 years (mean age 40) were studied. Epoetin (Eprex, Janssen-Cilag) was administered subcutaneously with the starting dose of 2000 IU three times per week for twelve months (range from 75 to 133 IU/kg/week, mean dose 102+/-21 IU/kg/week). Laboratory markers of: hematological response, iron status and erythrocyte metabolism were measured before epoetin administration. Afterwards the markers were controlled every three months. During epoetin treatment a significant increase in hemoglobin concentration was observed (100% patients responded in a positive way to epoetin). The following changes in erythrocyte metabolism were noticed: 1) in glycolytic enzymes: a significant increase in the activity of hexokinase and that of lactate dehydrogenase, 2) in glycolytic intermediates: a significant increase in the 2,3-diphosphoglycerate and adenosine triphosphate concentrations, 3) a significant increase sodium, potassium adenosine triphosphatase concentration, 4) the glucose uptake by erythrocytes significantly decreased while the lactate production remained stable. During anemia treatment with epoetin in hemodialysis patients not only quantitative but also qualitative changes in erythrocytes were observed.