Exercise intolerance is a common presentation of metabolic myopathies, especially of congenital errors of glycogen and lipid metabolism. Recently, however, exercise intolerance has been associated with specific defects in protein-coding genes of mitochondrial DNA (mtDNA), including mutations in genes for complex I, complex III, and complex IV. Contrary to the general rules of mitochondrial genetics, all patients were sporadic cases and all mutations were restricted to skeletal muscle, suggesting that they were somatic mutations not affecting the germ line.