Myoclonic epilepsy with ragged-red fiber syndrome has been associated with a mitochondrial DNA base substitution at nucleotide 8344 in the mitochondrial tRNA(Lys) gene. In several reported series, adult patients with these mutations have mild to moderate symptoms that progress slowly over many years. We describe a girl with MERRF syndrome who had an unusually rapid and severe clinical course, with onset of symptoms at age 7 years and death by age 14 years of overwhelming lactic acidosis. Postmortem tissue biopsy revealed variable but generally high percentages of mutant mitochondrial genomes in multiple organ systems. Twenty-one other members of her family were tested for the mutation and had varying percentages in leukocytes.