With conventional light and transmission electronmicroscopy we studied 10 cases of acoustic nerve tumour, 3 of which proved to be instances of von Recklinghausen neurofibroma and 7 of schwannoma. Schwannomas were not found to infiltrate the cochlear nerve. Hearing loss, if present in cases of schwannoma, could be related to non-specific lesions of the uninfiltrated cochlear nerve in the vicinity of the vestibular nerve tumour. Only neurofibromas were found to infiltrate the cochlear nerve. Distinction between tumour infiltration and non-specific lesion could be made by electron microscopy.