Drill-generated noise during chronic ear surgery, as well as surgical trauma, has been incriminated as a cause of sensorineural hearing loss in the operated ear. The contralateral ear is subjected to the drill noise but is spared the surgical trauma. Sixty-two patients who had undergone different mastoid operations served as the basis for this study. The average drilling time during surgery was 45 minutes. Sound pressure levels did not exceed 84 dB in the operated ear and 82 dB in the contralateral ear. Though some sensorineural hearing loss was found in the operated ear in fourteen patients, no changes in hearing were found in the contralateral ear. It is suggested that there is no damage exclusively due to the drill noise during mastoid surgery.