The histologic features of the endolymphatic sac and duct in 23 serially sectioned temporal bones with idiopathic or secondary endolymphatic hydrops were blindly compared with 22 randomly selected, normal temporal bones. In idiopathic hydrops, the pars rugosa of the endolymphatic sac extended out of the vestibular aqueduct into the dura in 29% of bones, compared with none of normal bones (P less than .01). In the other 71%, the pars rugosa in the vestibular aqueduct was surrounded by dura more commonly than normal. Functional studies are required to assess the relationship of these findings to hydrops. In secondary hydrops (eg, due to labyrinthitis), the endolymphatic duct was obliterated in the isthmus of the vestibular aqueduct by bone or fibrosis in seven of nine bones. Because of similar ossification and fibrosis elsewhere in the vestibular labyrinth, a direct relationship with hydrops cannot be assumed.