We devised a rapid PCR-based method to screen for an A----G transition at nucleotide 8344 of the human mitochondrial tRNA(Lys) gene, which was recently reported, by Shoffner and co-workers, to be associated with myoclonus epilepsy and ragged-red fibers (MERRF), a maternally transmitted mitochondrial encephalomyopathy (Shoffner et al. 1990). We confirmed this association in five of seven Italian MERRF pedigrees. The mutation was specific for the MERRF trait, because it was never found in mtDNA of non-MERRF individuals, including 14 normal and 110 diseased controls. Our study corroborates the idea that the A----G(8344) mutation is the most frequent and widespread genetic cause of MERRF.