Therapy with recombinant human erythropoietin (rhEPO) has become most valuable for the treatment of renal anemia in patients with various chronic renal diseases. For the first time this study presents data showing that rhEPO affects the metabolism of folic acid. There were 13 patients enrolled; they suffered from different chronic renal diseases and showed an impaired responsiveness to rhEPO therapy. Before starting rhEPO therapy the mean corpuscular volume of erythrocytes (MCV) was measured; MCV was 90.4 fl. During rhEPO therapy the MCV increased significantly by 14.8 fl (p < 0.05). The developing macrocytic anemia was overcome when folic acid was administered additionally for a mean period of 3.14 +/- 3 months. Hematocrit (Hct) also responded accordingly. Whereas Hct did not increase adequately during the exclusive treatment with rhEPO, an increase in Hct from 23 +/- 3.3 to 30 +/- 4.2% (p < 0.01) was observed after the addition of folic acid. These results are rather remarkable as folic acid serum levels were clearly within the normal range during the whole study period. So it can be concluded that rhEPO therapy results in an increased demand for folic acid. Even if serum concentrations are within the normal range, the administration of folic acid will enhance the effectiveness of rhEPO therapy so that the rhEPO dosage can be reduced.