Ferrritin can be measured in blood serum radioimmunometrically. Serum ferritin is directly correlated to body iron stores. In comparison to other parameters of storage iron (bone marrow iron, intestinal iron absorption) this quantitative diagnostic parameter is easily available. Thus it can be used to judge body iron status. In 20 patients with chronic haemorrhagic and 7 patients with posthaemorrhagic iron deficiency anaemia as well as nine blood donors with latent iron deficiency serum ferritin was used to control oral iron therapy. The continuous determination of serum ferritin during therapy gives a quantitative value of the relevant level of body iron stores. This value shows whether therapy was effective and when iron stores are replenished. The results demonstrate that oral iron therapy should be continued for at least 3 months from the time of normalisation of haemoglobin to obtain a sufficient restoration of iron depots.