The influence of body iron stores on the concentration of serum erythropoietin was studied in 48 hemodialyzed patients not receiving human recombinant erythropoietin, androgens or iron supplements. The serum erythropoietin concentration was 11.6 ± 10.4 mlU/ml. There was no correlation between the serum erythropoietin and the hematocrit or hemoglobin concentration; however, there was a correlation between the serum erythropoietin and the log of serum ferritin (r = -0.5699, p < 0.01). Serum erythropoietin levels were higher in the 18 ferropenic patients (serum ferritin < 50 ng/ml) than in the 30 patients with normal serum ferritin concentration (18 ± 13.8 vs. 7.8 ± 4.7 mlU/ml, p < O.01). The administration of intravenous iron to the ferropenic patients resulted in a reduction in serum erythropoietin independent of the respone of the anemia (18 ± 13.8 basal and 7.9 ± 6.5 mIU/mlat4weeks, p < 0.01). Our data would suggest that the concentration of erythropoietin in hemodialyzed patients is influenced by the serum ferritin l