We studied 10 patients with a variable degree of mtDNA depletion in muscle. Seven patients showed a clear-cut myopathic pattern, while the three remaining had brain involvement. There was no relationship between age at onset and relative mtDNA copy number in muscle, but we found an apparent correlation between clinical severity and degree of muscle mtDNA depletion. Muscle morphology showed that mtDNA depletion was associated with mitochondrial proliferation and cytochrome c oxidase negative fibers. Biochemical studies revealed single or combined defects of mtDNA-dependent respiratory chain complexes. Our data indicate that patients with mtDNA depletion may have a more variable age at onset and clinical evolution and wider phenotype than previously thought. The diagnosis of this condition, so far regarded as rare, may have been overlooked to some extent.