Two brothers presented with a clinical picture characterized by sideroblastic anemia, mild pancreatic Insufficiency and progressive muscle weakness. The presence of an associated permanent basal lactic acidemia raised the suspicion of a mitochondrial disease. A muscle biopsy performed In both siblings proved the presence of a significant number of ragged-red fibers, and respiratory chain enzymatic determinations demonstrated a reduced activity of complexes I, III and IV. Mitochondrial DNA studies disclosed the presence of multiple deletions both in skeletal muscle and, to a lesser extent, in leukocytes. Similar, but not identical deletions were also present in the leukocytes and muscle from their mother. Deletions were flanked by short direct repeats. We conclude that such patients suffer from a familial form of mitochondrial disease clinically resembling Pearson's syndrome, with a probable autosomal dominant inheritance.