Despite the progress in the knowledge of iron metabolism, its precise assessment remains uneasy. Serum ferritin assesses the extent of storage iron. Serum iron and the percentage of transferrin saturation evaluate the tissues' iron supply. But these parameters are indirect measurements and they do not reflect marrow iron supply. Serum transferrin receptors, red cell ferritin and red cell zinc protoporphyrin are good indicators of this iron supply to the erythroid marrow for erythropoiesis. Since the introduction of recombinant human erythropoietin, it has become apparent that an adequate iron supply to the bone marrow is essential for a satisfactory hematopoietic response. In some cases, despite a high baseline ferritin, iron may not be sufficiently released from reserves in the bone marrow, resulting in a functional iron deficiency. The percentage of hypochromic red cells and reticulocyte haemoglobin content tends to reflect direct marrow iron status.