Some surgeons have shown that tumors of the internal auditory canal and cerebellopontine angle may be removed with preservation of hearing through the suboccipital approach. If hearing is to be conserved, the cochlear division of the Vlllth cranial nerve and blood supply of the labyrinth must be preserved. In addition, surgical entry into the labyrinth, upon removal of the posterior wall of the internal auditory canal, must be avoided since it is likely to result in permanent sensorineural hearing loss. Careful anatomic dissection of 20 human temporal bones has shown that exposure of the lateral-most recess of the internal auditory canal from a suboccipital approach is impossible without injury to the endolymphatic duct, common crus, vestibule or ampulla of the posterior semicircular canal. Previous authors have suggested that exposure of the horizontal crest may be used as a safe landmark in avoiding labyrinthine injury. However, our study has shown that exposure of the horizontal crest usually leads to labyrinthine injury. In 19 out of 20 cases, the labyrinth would have been entered had the horizontal crest been used as a landmark for the lateral limit of bone removal. The application of the anatomical relationships quantified in this study may improve our ability to avoid labyrinthine injury in the suboccipital removal of acoustic neuromas.