Mitochondrial dysfunction is an important cause of human deafness, implicated in genetic deafness, toxin and noise damage. We assessed the mechanism of cochlear dysfunction in a population of 11 subjects with a specific mitochondrial disorder caused by the A3243G mitochondrial DNA mutation. Psychophysical tests were carried out to assess the inner and outer hair cell functions in vivo. Inner hair cell function was assessed using a measure of hearing threshold in the presence of "threshold-equalizing noise" which can indicate "dead regions" where the transduction mechanism fails. Outer hair cell function was assessed by using the notched-noise method to measure auditory filter width, dependent on active mechanisms in the outer hair cell. The results support the conclusion that this mitochondrial disorder causes both inner and outer hair cell dysfunctions. Evidence of inner hair cell dysfunction was found mainly in basal (high frequency) regions of the cochlea and occurred even in some subjects with only mild hearing loss. Evidence of outer hair cell dysfunction was found in some instances where pure tone threshold was at or close to normal. The common occurrence of dead regions in the basal cochlea has treatment implication for this form of deafness; such people may not be helped by amplification of high frequencies.