The level of serum ferritin is a reliable indicator of body iron stores. Exceptions include liver disease, malignant diseases, and treatment of iron-deficiency anemia. The latter was noted in iron- deficient infants who showed a rise of serum ferritin to normal levels in the first week of treatment. To evaluate this in adults, 14 patients with iron-deficiency anemia were studied prior to and after beginning treatment with oral ferrous sulfate in standard dose, 300 mg t.i.d., or double dose, 600 mg t.i.d. Serum ferritin was assayed by radioimmunoassay. No rise occurred in the first 3 wk in 5 patients treated with standard dose, although hematologic response occurred. With double dose, 7 of 9 showed a ferritin rise in 2 days with return to subnormal levels within 6 days of discontinuing iron. This study indicates that standard treatment of iron deficiency anemia in adults does not cause a rise in serum ferritin until hemoglobin levels are normal. The early rise seen with double dose is most likely due to absorption of iron in excess of utilization for erythropoiesis resulting in temporary storage. When iron is discontinued, stores are rapidly depleted as reflected by the prompt decrease in serum ferritin.