Plasma ferritin is a secretory component of intracellular ferritin synthesis. In normal persons its amount reflects the size of iron stores. A decrease to less than 12 mug per liter indicates iron deficiency. Increased iron stores are associated with an increased plasma ferritin level. Various other conditions, however, can increase the plasma ferritin concentration including increased metabolism, inflammation, tissue damage and neoplastic disease. The use of the plasma ferritin determination in diagnosing iron overload depends on excluding these other causes, leaving storage iron as the only explanation for the increased plasma ferritin. It is then necessary to establish the parenchymal nature of the iron overload by showing an elevated transferrin saturation and, if elevated, the more definitive liver biopsy should be done.