Intraoperative brain stem auditory evoked potential (BAEP) and somatosensory evoked potential (SEP) monitoring was evaluated in 16 patients each undergoing intracranial operation for basilar artery aneurysm. The 16 patients had 18 posterior circulation aneurysms, including 2 patients with 2 aneurysms. Fourteen aneurysms arose from the rostral basilar artery, 2 arose from the midbasilar artery, 1 arose from the vertebrobasilar junction, and 1 arose from the proximal segment of the posterior cerebral artery. Five aneurysms were classified as giant (i.e., greater than 25 mm), and 5 aneurysms were large (i.e., 15 to 25 mm). Ten patients had BAEP and SEP monitoring, 4 had BAEP monitoring only, and 2 had SEP monitoring only. Two patients showed significant abnormalities during operation, including 1 patient with transient changes in the BAEP when the lower pons and the 8th cranial nerve were retracted. Another patient had progressive increases in latency and decreases in amplitude and subsequent loss of the SEP cortical components during a period of intermittent temporary rostral basilar artery occlusion. Wave P13 was also lost during that period. The cortical components as well as Wave P13 returned after circulation was restored. The BAEPs were unchanged in the same patient during the period of temporary basilar artery occlusion. Fourteen patients had no significant abnormalities. There were no consistent changes during the various stages of operation. BAEP and SEP monitoring failed to identify ischemic events in 4 patients with neurological findings of brain stem ischemia immediately after operation (i.e., 25% false-negative studies).(ABSTRACT TRUNCATED AT 250 WORDS)