We describe a novel mutation in the mitochondrial tRNA(Ile) gene, an A to G transition at nucleotide position 4267, in a 37-year-old woman with myopathy, ataxia and sensorineural hearing loss. The A4267G mutation was heteroplasmic in several of the proband's tissues and single fibre analysis revealed significantly higher levels of mutated mitochondrial DNA in cytochrome c oxidase-deficient fibres than cytochrome c oxidase-positive fibres. It is predicted to disrupt a highly conserved base pair within the aminoacyl acceptor stem of the tRNA causing functional impairment, and as such fulfils all the accepted criteria for pathogenicity. Moreover, we were unable to detect the A4267G mutation in lymphocytes, buccal epithelia and hair of the patient's mother and two siblings, implying that the A4267G transition represents a sporadic, germline mutation.