Assessment of iron status is important, because iron deficiency and overload have pathologic consequences. Serum ferritin, the function of which is unknown, is frequently used to assess labile iron stores for the purpose of ensuring their adequacy for erythropoiesis. However, measurable ferritin levels can be increased when tissue ferritin is released during cellular injury, and erythropoietic blockade can increase the labile iron pool, elevating serum ferritin levels despite suppressed erythropoiesis. Erythropoietin-stimulated red blood cell (RBC) production can quickly decrease the labile cellular iron pool and reduce serum ferritin, unless supplemental iron is supplied. A serum ferritin level less than 12 microg/L indicates that there is no iron in the stores (absolute iron deficiency), and levels above 15 microg/L may still not be sufficient to meet erythropoietic demand. This is particularly true in patients receiving erythropoietin, in which the stimulated erythropoiesis requires extra iron supplies. Because of the limitations of serum ferritin measurements and questions regarding the effects of free iron, clinicians need not be too alarmed by high serum ferritin levels. At the very least, safety concerns must be balanced against the real need for iron supplementation to maintain adequate erythropoiesis.