The sideroblastic anemias are characterized by ring sideroblasts, that is, red cell precursors with mitochondrial iron accumulation. We therefore studied the expression of mitochondrial ferritin (MtF) in these conditions. Erythroid cells from 13 patients with refractory anemia with ring sideroblasts (RARS) and 3 patients with X-linked sideroblastic anemia (XLSA) were analyzed for the distribution of cytoplasmic H ferritin (HF) and MtF using immunocytochemical methods. We also studied 11 healthy controls, 5 patients with refractory anemia without ring sideroblasts (RA), and 7 patients with RA with excess of blasts (RAEB). About one fourth of normal immature red cells, mostly proerythroblasts and basophilic erythroblasts, showed diffuse cytoplasmic positivity for HF, but very few were positive for MtF (0%-10%). Similar patterns were found in anemic patients without ring sideroblasts. In contrast, many erythroblasts from patients with sideroblastic anemia (82%-90% in XLSA and 36%-84% in RARS) were positive for MtF, which regularly appeared as granules ringing the nucleus. Double immunocytochemical staining confirmed the different cellular distribution of HF and MtF. There was a highly significant relationship between the percentage of MtF⁺ erythroblasts and that of ring sideroblasts (SpearmanR = 0.90; P < .0001). Reverse transcription-polymerase chain reaction studies demonstrated the presence of MtF mRNA in circulating reticulocytes of 2 patients with XLSA but not in controls. These findings suggest that most of the iron deposited in perinuclear mitochondria of ring sideroblasts is present in the form of MtF and that this latter might be a specific marker of sideroblastic anemia.