Object. Postoperative hearing loss is the most serious complication of microvascular decompression for facial spasm and trigeminal neuralgia. To test the usefulness of brainstem auditory evoked potential (BAEP) as a intraoperative monitoring, in relation to hearing loss we compared BAEP records and postoperative hearing. We also compared BAEP data during microvascular decompression to that obtained during acoustic tumor surgery.Methods. BAEP was monitored every 5 to 10 minutes with a loudness of 100dB and 75Hz per second of frequency. 15 cases of trigeminal neuralgia and 10 cases of facial spasm were monitored during microvascular decompression. 8 cases of acoustic tumor were monitored intraoperatively with BAEP using the same method used with microvascular decompression and the results were compared.Results. Mean delay of the Vth wave latency was 1.16 msec for facial spasm and 1.15 msec for trigeminal neuralgia. Both were significantly longer than the delay found in acoustic tumor removal, in which hearing was preserved postoperatively. However, hearing disturbance was found in two cases of facial spasm, one severe case of hearing loss and another mild hearing impairment. No hearing impairment was seen in trigeminal neuralgia cases.Conclusions. Delay of Vth wave latency occurred equally in surgery for facial spasm and trigeminal neuralgia, yet hearing disturbance was found only in surgery for facial spasm. Delay of BAEP may be caused by the retraction of the cochlear nerve, but hearing disturbance might be caused by direct manipulation injury including inflammation and microcirculatory impairment to the cochlear nerve which may occur in surgery for facial spasm and may not be detected by intraoperative BAEP monitoring.