Backround The endolymphatic sac is an organ devoid of sensory receptors. It is connected with the endolymphatic compartment and contains endolymph. Endolymphatic sac tumor is a rare neoplasm involving the middle and inner ear described in humans and dogs, does not show cellular characteristics of malignancy, but can be locally invasive and involve destruction of the temporal bone and adjacent structures. Case description: An 8 months old female cat was referred because of sudden onset of vestibular signs starting 3 days prior to referral. On clinical examination the patient showed depression, right head-tilt, left sided facial paralysis, horizontal nystagmus with fast phase to the left. The magnetic resonance images showed a voluminous extra axial lesion, with irregular morphology and well defined margins, with intracranial extension in the region of the pons, rostral medulla oblongata, cerebellar vermis, floccule and left cerebellar hemisphere. Due to the progressive clinical deterioration, the cat was euthanized two weeks later. A necropsy was then performed and histological samples taken. The necropsy revealed the presence of a voluminous dark red irregular mass extending from the tympanic bulla to the posterior cranial fossa following the left glossopharyngeal nerve. The histopathologic exam of the extra-axial lesion featured a non-encapsulated, moderately cellular, rather loose, proliferation of cuboidal to columnar epithelium breaching through chunks of an otherwise normal appearing dura mater and invading some cranial nerves.
 Sections of the cerebellum and brainstem revealed moderate, focal, impingement of the parenchyma with very mild extension of the proliferating cells into the ventral left side of the medulla oblongata.
 Based on these histological characteristics, the lesion was defined as endolymphatic sac tumor, a rare neoplasm described in human beings and with 2 reports in dogs.

 Conclusion To our knowledge, this is the first report describing an endolymphatic sac tumor in a cat.