Presbycusis is a histologically and genetically heterogenous group of disorders, which lead to progressive, primarily sensorineural hearing loss with aging. Acquired mitochondrial DNA defects have been proposed as important determinants of aging, particularly in neuro-muscular tissues. The spiral ganglion and membranous labyrinth from archival temporal bones of 5 patients with presbycusis were examined for mutations within the mitochondrially-encoded cytochrome oxidase II gene. When compared to controls, results indicate that mitochondrial mutations in the peripheral auditory system occur commonly with age-related hearing loss, that there is great individual variability in both quantity and location of mutation accumulation, and that at least a proportion of presbycusis patients have a highly significant load of mutations in auditory tissue. This work supports the hypothesis that acquired mitochondrial mutations are a determinant of hearing loss in a subgroup of presbycusis patients.