We describe a patient with acute combined demyelinating disease of the central and peripheral nervous systems associated with the A8344G mutation in the mitochondrial tRNA lysine gene. A 7-year-old boy presented with acute onset of palpitations, tinnitus, ataxia, bilateral sixth nerve palsy, and flaccid quadriparesis. Serum creatine kinase and lactate were mildly increased. Electromyography showed demyelinating sensory and motor polyneuropathy. Brain magnetic resonance imaging demonstrated demyelination in the left thalamus and magnetic resonance spectroscopy revealed a lactate peak corresponding to this lesion. Histologic analysis of the muscle showed cytochrome c-oxidase-deficient fibers and ragged red fibers. Respiratory chain analyses revealed deficiencies of complexes I and IV. Molecular genetic analyses of the muscle showed an A8344G (MERRF) mutation in mitochondrial tRNA lysine. To the best of our knowledge, this is the first description of this mutation associated with acute combined demyelinating disease of the central and peripheral nervous systems.