In the recombinant human erythropoietin (rHuEPO) treatment of renal anemia, the target level has widely been accepted as approximately 30% of hematocrit (Hct) which means only partial improvement of the anemia. We tried to maintain 1 female and 9 male patients within a normal Hct and to estimate their exercise tolerance abilities. During the study, no serious complications were experienced. Their maximal oxygen uptake (VO2max), evaluated for 1-6 months after reaching the desired Hct of 35-40% with rHuEPO, was 30.7 +/- 2.4 ml of O2/kg/min. These results were significantly higher than those of the control groups whose Hct levels were maintained at around 30%. Although 1 female and 2 male patients were considered to be within the normal exercise tolerance abilities, the others showed lower values in comparison with the Japanese age- and sex-matched values. Thus, it is not always possible to regain full exercise tolerance abilities even after restoring a normal Hct. Unknown factors besides anemia impairing their exercise tolerance abilities might exist in chronic dialysis patients.