The endolymphatic sacs are described in temporal bone specimens from a 31-year-old man with bilateral Mondini disorder. The ducts and sacs are thin-walled, cyst-like structures with complete absence of loose vascular perisac tissue, and are directly apposed to the bone of the vestibular aqueduct. Histological evidence of severe bone erosion is present in these specimens and is most marked in the intermediate and distal portions of the vestibular aqueduct. It is also present in the foveal region of the posterior temporal bone surface underlying the sac. Erosion of the bony wall of the paravestibular canaliculus (PVC) is demonstrable, with incorporation of the vein of the PVC inside the margin of the widened vestibular aqueduct. These findings suggest a causal relationship between pressure within the endolymphatic duct and sac and erosion of the surrounding bone. The absence of endolymphatic hydrops of the cochlea and vestibular organs in the Mondini disorder contrast significantly with the endolymphatic hydrops seen in Meniere's disease.