Interferon has been shown to be an effective treatment of congenital dyserythropoiesis type I (CDA-I), but the optimal dose and the feasibility of this treatment remains to be determined. Here, in a 9-yr follow-up of a single patient, we show that interferon remains active during such a long period. The optimal dose of conventional alpha interferon could be evaluated at 2 million units twice a week. Pegylated interferon could be used as well at a dose of 30 microg/wk. During interferon treatment, serum and erythrocyte ferritin levels decreased progressively, and remained inversely correlated with haemoglobin levels. On repeated liver biopsies, iron overload could be normalized. Low dose interferon is a long-term treatment of CDA-I, and allows a significant decrease in iron overload, that could be interesting even in patients who are only moderately anaemic.