Hereditary non-syndromic deafness has been associated with a point mutation in the mitochondrial 12S rRNA gene. We present data from deaf individuals in 12 nuclear families originating from a small village in Zaire. The patients have a sudden-onset and profound, bilateral sensorineural deafness with a highly variable age of onset. Inheritance is compatible with a mitochondrial DNA defect. Sequencing of the mitochondrial 12S rRNA gene revealed the presence of a homoplasmic 1555 A to G mutation in the patients and their normal siblings. The mutation is invariably associated with a T to C transition at 1420 in the same gene. Additional (mitochondrial or autosomal) genetic defect(s) or an environmental factor must be implicated in the expression of the defect. In Epstein-Barr-virus-transformed lymphocytes harbouring the normal or mutant mitochondrial DNA, no differential effect of aminoglycosides on protein translation was observed.