HYPOTHESIS: Cochlear pathology can be evaluated in living animals using optical coherence tomography (OCT). BACKGROUND: The current imaging methods available for the detailed analysis of cochlear pathology in a clinical setting provide only limited information. Thus, a cochlear imaging modality with high definition is needed for improving the diagnosis of cochlear pathology. OCT has been used in other fields for obtaining high-resolution subsurface images, and its use could potentially be extended to the analysis of cochlear pathogenesis. METHODS: Slc26a4(-/-) mice, which generate endolymphatic hydrops, and their littermates were used in this study. Auditory function was monitored by the auditory brainstem responses (ABR). After the mice were placed under general anesthesia, OCT images of the cochlea were captured. The cochlea was subsequently dissected out and histologically evaluated. Three or 7 days later, the wild-type mice cochleae were visualized again. RESULTS: In ABR assessments, Slc26a4(-/-) mice showed severe hearing loss, while no significant hearing loss was found in Slc26a4(+/-) or Slc26a4(+/+) mice. OCT demonstrated normal morphology in the cochlea of both Slc26a4(+/-) and Slc26a4(+/+) mice, including the location of Reissner's membrane. Meanwhile, in Slc26a4(-/-) mice, obvious dislocation of Reissner's membrane was observed, indicating severe endolymphatic hydrops. These findings in the OCT images were consistent with the histologic results for the cochlear morphology, as observed with hematoxylin and eosin staining. Three or 7 days later, wild-type cochleae were successfully visualized using OCT, and no otitis media or labyrinthitis was observed. CONCLUSION: OCT can be applied in the detection of endolymphatic hydrops in living mice, indicating the potential of OCT for cochlear imaging analyses for clinical use in the near future.