Composed of a postmitotic stable tissue, the inner ear is a target organ for mitochondrial DNA (mtDNA) mutation. To determine whether mtDNA mutation is a predisposing factor in patients with sensorineural hearing loss (SNHL), the authors assessed the mtDNA4977 deletion from 60 patients with SNHL and 47 normal control subjects. All cases had no past history of ototoxic or noise exposure, middle ear disease, or other known etiological factors for SNHL. DNA specimens extracted from peripheral blood leukocytes were used for detection of mtDNA4977 deletion by polymerase chain reaction. Patients with SNHL had a significantly higher rate of the mtDNA4977 deletion than did controls (75% vs. 30%, P < 0.0001). The detection rate of mtDNA4977 deletion was significantly increased with the deterioration of the hearing threshold. Aging did not influence the detection rate of mtDNA4977 deletion in either the control or SNHL group. The authors have described high detection rates of the mtDNA4977 deletion in patients with idiopathic bilateral SNHL and propose that at least some of the advanced SNHL cases should be categorized as mitochondrial oxidative phosphorylation diseases. This inference would offer novel possibilities for treatment and prevention of SNHL including presbycusis.