SUMMARYThe diagnosis of acoustic neuroma depends upon expansion of the internal auditory canal; two cases are described where trispiral tomography appeared to show an expanding lesion but CT demonstrated an enlarged canal containing only normal structures.ABSTRACTThe empty pituitary fossa is well recognised. Petrous block tomography in patients referred for exclusion of acoustic neuroma has on three occasions demonstrated an apparently expanded canal, with suggestion of bone erosion, implying an expanding lesion, but C.T. with air has shown only normal structures in an otherwise empty canal.