The authors describe a 47-year-old man who presented with proximal muscle weakness, myalgia, elevated creatine kinase, and features of a pure myopathic syndrome in whom they have identified a novel mutation in the mitochondrial tRNA(Ala) gene. This 5591G>A transition is heteroplasmic, segregates with cytochrome c oxidase deficiency in single muscle fibers, and fulfills recognized criteria for pathogenicity. This case exemplifies the wide-ranging clinical spectrum of mitochondrial disease presentations.