Recent studies have identified a mitochondrial DNA mutation (1555G) which causes sensorineural hearing loss (SNHL). In many cases deafness follows exposure to aminoglycoside antibiotics, the 1555G mutation sensitizing the inner ear to these drugs. The 50 cases reported to date are discussed, as are the possible mechanisms behind the pathogenesis of this mutation. This finding in families from a wide range of ethnic backgrounds suggests that the 1555G mutation is one of the more common genetic causes of SNHL and provides a fascinating example of how a genetic mutation interacts with an environmental factor with harmful effect.