Ferritin in serum was quantitated by radioimmunoassay to determine the usefulness of this assay in reflecting iron stores during normal development and in the diagnosis of iron deficiency and iron overload in infants and children. In the adult, serum ferritin has been reported to correspond to the magnitude of iron stores throughout a wide range. In 573 normal infants and children, we found the concentration of serum ferritin to parallel known changes in iron stores during development. The median serum ferritin concentration was 101 ng/ml at birth, rose to 356 ng/ml at 1 mo of age, and then fell rapidly to a median value near 30 ng/ml (95% confidence limits: 7-142 ng/ml) between 6 mo and 15 yr of age. In the adult, median concentrations diverged according to sex, 39 ng/ml in the female and 140 ng/ml in the male. In 13 children with iron-deficiency anemia, the serum ferritin concentration was 9 ng/ml or less. Overlap with the normal population was small, and no conditions were found to give "false" low values. In children with β-thalassemia major and sickle cell anemia, median values were elevated to 850 and 160 ng/ml, respectively. Possibly misleading, elevated values were obtained in some patients with acute infection and in acute lymphoblastic leukemia. The serum ferritin assay promises to be a useful tool in the evaluation of iron status, particularly in children.