Bone marrow iron supply may become rate limiting for hemoglobin synthesis during rhEPO-stimulated erythropoiesis. In the present study we followed the occurrence of hemoglobin-deficient red cells as a parameter of iron-deficient erythropoiesis in rhEPO-treated dialysis patients. rhEPO-treated patients with iron overload displayed very low numbers of hypochromic red cells (1%), while those with iron-deficiency had a hypochromic subpopulation of 22% (normal range < 2.5% of circulating red cells). Prior to rhEPO treatment, 10 dialysis patients showed normal numbers of hypochromic red cells (2.1%), despite mild iron deficiency (transferrin saturation: 17%). Once rhEPO (150 U/kg/week) was started, the percentage of hypochromic red cells rose significantly to 15.3% within 4 weeks of therapy. This was readily reversed when intravenous iron (750 mg/4 weeks) was added to the therapeutic regimen (5.5% after 4 weeks of i.v. iron). Taken together, quantitative red cell analysis seems to be a reliable tool to detect iron-deficient erythropoiesis in rhEPO-treated dialysis patients.