A rare point mutation at nucleotide position 8356 in the transfer RNA gene in mitochondrial DNA was found in a Japanese family. Our proband had migraine and dementia associated with lactic acidosis in addition to myoclonic epilepsy with ataxia and ragged-red fibres in a muscle biopsy specimen consistent with the clinical characteristics of myoclonic epilepsy with ragged-red fibres (MERRF). His mother, who had the same point mutation, also had migraine but without myoclonus or ataxia. His aunt, who had the same point mutation and migraine, developed diabetes mellitus, encephalomyopathy and several stroke-like episodes associated with lactic acidosis (MELAS). This is the third family with the rare mutation seen in American and Italian families. The mutation may not be specific to Caucasians, and is probably closely related to the MERRF/MELAS overlap syndrome.