A mutation at base pair (bp) 3243 in mitochondrial DNA has been associated with mitochondrial myopathy, encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS). A mutation at bp 8344 has been described as the cause of myoclonic epilepsy and ragged-red fiber disease (MERRF). Mitochondrial DNA was analyzed in a family with symptoms and signs consistent with MERRF. The DNA regions flanking bp 3243 and bp 8344 were amplified using the polymerase chain reaction, and the products were digested with restriction enzymes. The MELAS mutation at bp 3243 was found, but not the mutation at bp 8344. This illustrates the diverse clinical manifestations of the MELAS mutation.