We studied the significance of free erythrocyte protoporphyrin (FEP) in relation to iron status, aluminum levels and anemia in uremic patients on chronic dialysis. All but 1 patient showed high FEP values closely related to the degree of anemia. Increased FEP levels are due to a defective heme synthesis, not related to iron deficiency or aluminum overload. Treatment of anemia with recombinant human erythropoietin reduced FEP values. We therefore hypothesize that recombinant human erythropoietin ameliorates an enzymatic defect in heme synthesis.