Iron and erythropoiesis are inextricably linked. Erythropoiesis is a dynamic process that requires 30-40 mg of iron per day. In normal circumstances this is met from red cell destruction but in anaemia this will not be the case. Reduced iron stores will limit iron supply to erythroblasts but normal or raised iron stores may not be able to supply iron fast enough. This is particularly true when the marrow is stimulated by erythropoietin therapy; the most common cause of failure to respond is "functional iron deficiency"'. This entity can only be effectively addressed by intravenous iron therapy. While haemoglobin and serum ferritin concentrations reflect the major iron pools, iron supply to erythroid cells can only be assessed by measuring effective haemoglobinization through the percentage of hypochromic red cells in the circulation.