In tympanoplasty, trauma to the cochlea is usually caused by the transmission of vibration induced during the removal of cholesteatoma, granulation tissue, and tympanosclerosis surrounding the ossicular chain. In 100 sequential cases 20 showed evidence of cochlear trauma. Hearing was recovered in 10 of these cases during the immediate postoperative period. In nine the hearing was partially recovered but remained below the preoperative bone-conduction level, and in one there was a total cochlear hearing loss. When the stapes footplate was fractured or dislocated and a perilymphatic fistula developed, profound cochlear hearing loss occurred. This complication usually can be avoided by inspection of the round and oval windows under high magnification and by tissue grafts over the fistula. An unanticipated pathologic fistula of the semicircular canals is another common source of major cochlear damage. Despite all precautions and sometimes without apparent cause, a total cochlear hearing loss may result after a tympanoplasty. Although this is rare, as evidenced by the fact that there were only 15 cases in some 1000 tympanoplasties, before surgery the surgeon must inform the patient of the risks involved.