We looked for the A-->G transition at position 8344 of mtDNA in 150 patients, most of them with diagnosed or suspected mitochondrial disease, to assess the specificity of this mutation for the MERRF phenotype, to define the clinical spectrum associated with the mutation, and to study the relationship between percentage of mutation in muscle and clinical severity. Our results confirm the high correlation between the A-->G transition at position 8344 and the MERRF syndrome, but they also show that this mutation can be associated with other phenotypes, including Leigh's syndrome, myoclonus or myopathy with truncal lipomas, and proximal myopathy. The absence of the mutation in four typical MERRF patients suggests that other mutations in the tRNA(Lys) gene, or elsewhere in the mitochondrial DNA, can produce the same phenotype.