Pre-, per- and post-operative ECoG findings obtained from a 43-year-old woman with an acoustic neuroma were studied. Although CAP presented a normal waveform in response to click during the whole process of the operation, it was transformed into an abnormally broadened negative waveform in shape with a mild hearing exacerbation, and has remained unchanged up to the present. This broad response was considered to be a receptor potential, because it showed no adaptational amplitude reduction. Additionally, postoperative ECoG to tone burst stimuli demonstrated an increase in DC potential following the stimulus envelope. Consequently, the broad negative response to click was considered to be mainly composed of an enlarged negative SP. The generation mechanism underlying the phenomenon of an abnormally increased negative SP found after the excision of an acoustic neuroma is discussed, with reference to several items in the literature. Similar responses in the postoperative ECoG of patients with acoustic neuromas obtained by us in the past were given as additional examples. CAP as an indicator for intraoperative monitoring can hardly predict postoperative auditory function. It is to be hoped that a more reliable auditory monitoring method during the operation will be established.