Maternally transmitted non–syndromic deafness was described recently both in pedigrees with susceptibility to aminoglycoside ototoxicity and in a large Arab–Israeli pedigree. Because of the known action of aminoglycosides on bacterial ribosomes, we analysed the sequence of the mitochondrial rRNA genes of three unrelated patients with familial aminoglycoside–induced deafness. We also sequenced the complete mitochondrial genome of the Arab–Israeli pedigree. All four families shared a nucleotide 1555 A to G substitution in the 12S rRNA gene, a site implicated in aminoglycoside activity. Our study offers the first description of a mitochondrial rRNA mutation leading to disease, the first cases of non–syndromic deafness caused by a mitochondrial DNA mutation and the first molecular genetic study of antibiotic–induced ototoxicity.