In recent years, technical progress has created new complex acoustic implants which send an electrical stimulus to the eighth cranial nerve through one or more electrodes inserted through the round window into the scala tympani of the cochlea. The abnormal--mostly osteosclerotic--processes which cause deep hearing loss may prevent electrode insertion. Therefore, internal ear anatomy must be detailed, which is essential to assess the feasibility of surgery and, if surgery is indicated, to plan it properly. High resolution CT (HRCT) was performed on 79 patients to study cochlear patency, round window shape and patency, degree of temporal bone pneumatization and the proximity of vascular structures (carotid artery and jugular vein). On the basis of HRCT results, 14 of 79 patients were excluded from surgery. Comparing HRCT with surgical findings, the authors conclude that HRCT is the method of choice to examine the candidates to cochlear implant thanks to its high spatial resolution and excellent depiction of even the smallest structures. Its only limitation is that it fails to assess the lack of patency of the cochlear canal due to fibrosis, which is not associated with demonstrable density changes (3 of 19 surgical patients). This problem may be solved by submitting the potential surgical candidates to MRI.