Cytochrome b mutations are rare causes of exercise intolerance. We report an 18-year-old man with exercise intolerance since childhood, resting lactic acidosis, cytochrome c oxidase (COX)-positive ragged-red fibers, and isolated muscle complex III deficiency due to a heteroplasmic m.14849T>C mutation in cytochrome b. We review previously described patients carrying mutations in the same gene. COX-positive ragged-red fibers together with exercise intolerance and lactic acidemia provide a clue for the diagnosis of this rare mitochondrial disorder.