BACKGROUND: With the use of standard electrodes in cochlear implantation, residual acoustic hearing is markedly reduced or even lost. Possible reasons for this loss are direct implantation trauma to the inner ear, reaction of the cochlea triggered by the implantation, and change of cochlear mechanics due to the electrode. HYPOTHESIS: The introduction of a cochlear implant electrode does not alter the global mechanical behavior of the cochlear fluid as recorded at the stapes and the round window (RW) to the point of clinical relevance. OBJECTIVES: 1) To assess RW motion in response to acoustic stimulation in live human subjects for the first time and to compare findings with the published data on similar measurements in human temporal bones; 2) to test the hypothesis by comparing intraoperative measurements of the stapes with the RW before and after cochlear implant. PATIENTS AND METHODS: The study included 18 adult patients with profound bilateral hearing loss. A scanning laser Doppler interferometer system measuring through the facial recess and a calibrated multi-sinewave acoustic stimulation tone in the ear canal were used. Changes in cochlear mechanics were assessed by comparing intraoperative measurements of stapes with RW membrane responses to acoustic signals before and after cochlear implantation. RESULTS: Vibration amplitudes and phase at the stapes and RW were not different in our patient group from published results from temporal bones. No significant changes in amplitude and phase were seen at the stapes and RW after cochlear implantation. CONCLUSION: It was possible to assess RW motion in live human subjects. Our results provide evidence that even a standard electrode does not alter cochlear mechanics to the point of clinical relevance.