Mitochondrial iron overload in acquired idiopathic sideroblastic anemia (AISA) may be attributable to mutations of mitochondrial DNA (mtDNA), because these can cause respiratory chain dysfunction, thereby impairing reduction of ferric iron (Fe3+) to ferrous iron (Fe2+). The reduced form of iron is essential to the last step of mitochondrial heme biosynthesis. It is not yet understood to which part of the respiratory chain the reduction of ferric iron is linked. In two patients with AISA we identified point mutations of mtDNA affecting the same transmembrane helix within subunit I of cytochrome c oxidase (COX I; ie, complex IV of the respiratory chain). The mutations were detected by restriction fragment length polymorphism analysis and temperature gradient gel electrophoresis. One of the mutations involves a T --> C transition in nucleotide position 6742, causing an amino acid change from methionine to threonine. The other mutation is a T --> C transition at nt 6721, changing isoleucine to threonine. Both amino acids are highly conserved in a wide range of species. Both mutations are heteroplasmic, ie, they establish a mixture of normal and mutated mitochondrial genomes, which is typical of disorders of mtDNA. The mutations were present in bone marrow and whole blood samples, in isolated platelets, and in granulocytes, but appeared to be absent from T and B lymphocytes purified by immunomagnetic bead separation. They were not detected in buccal mucosa cells obtained by mouthwashes and in cultured skin fibroblasts examined in one of the patients. In both patients, this pattern of involvement suggests that the mtDNA mutation occurred in a self-renewing bone marrow stem cell with myeloid determination. Identification of two point mutations with very similar location suggests that cytochrome c oxidase plays an important role in the pathogenesis of AISA. COX may be the physiologic site of iron reduction and transport through the inner mitochondrial membrane.