A longitudinal study has been conducted on serum ferritin concentrations in children with thalassemia major treated with desferrioxamine (DFO), both intramuscularly and by continuous subcutaneous infusion, in order to evaluate the time interval after which iron chelation becomes effective. In a first group of 19 children, treated intramuscularly with DFO at a dose of 20 mg/kg for 20 days a month, ferritin levels were followed for 739 days and a progressive increase with a significant linear correlation with time was seen. The daily increase was calculated to be 8.53 ± 1.95 ng/ml. In the second group of 12 children treated with DFO by continuous subcutaneous infusion (≥ 20 mg/kg/day for 6 days a week), the increase in ferritin was markedly lower, the mean daily value being 4.77 ± 3.30 ng/ml. Only after the first 360 days of treatment did the serum ferritin decrease, by 1.95 ± 3.32 ng/ml/day, in spite of a mean daily iron contribution from blood transfusions of 7.09 ± 2.69 mg/day. These changes in serum ferritin led us to conclude that one can only expect to obtain a decrease in the body’s iron overload after more than a year of treatment with subcutaneous DFO.