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Sponsorships or competing interests that may be relevant to content are dis-closed at the end of this article. Objectives. To characterize mitochondrial sequence variants present in a nationwide hereditary deafness DNA reposi-tory of samples from deaf subjects and to define the clinical presentation and audiometric characteristics of individuals with a mitochondrial sequence variant. Study Design. Retrospective review of results for select mito-chondrial mutations performed on DNA samples from sub-jects compiled from 1997 to 2009. Setting. National hereditary deafness DNA repository. Subjects and Methods. Available samples from subjects in the repository were screened to identify those with mitochondrial sequence variants. Clinical data on the nature of mutation, type and severity of the hearing loss, and sex, age at diagnosis, family history of hearing loss, and ethnicity were analyzed. Results. Eighty-six patients were identified with mitochondrial mutations or 3.5 % of the subjects studied. Among those with mitochondrial mutations, 21 (24.4%) had the m.7445A.G sub-stitution, 18 (20.9%) had the m.1555A.G substitution, 18 (20.9%) had the m.961T.G substitution, and 29 (33.7%) had a m.961delT.C(n) complex deletion. The majority of patients had bilateral severe to profound hearing loss. Fifty-three (62%) patients were female, and a family history of hearing loss was documented in 69 (80.2%) patients. The deafness was recog-nized prior to 3 years of age in 26 patients. Conclusion. Mitochondrial deafness in this sample was associ-ated with a variety of genetic mutations and a wide spec-trum of clinical presentations. Because of increased aminoglycoside susceptibility associated with some forms of mitochondrial deafness, matrilineal relatives may be at risk in those cases, highlighting the importance of making an accurate diagnosis prior to exposure