Brainstem auditory evoked responses (BAERs) were recorded from 202 patients with definite, probable, or possible multiple sclerosis (MS). Definitions of abnormality were based only on interwave separations and the wave I/wave V amplitude ratio. Thirty-two percent of the patients had abnormal BAERs, and the presence of clinically unsuspected lesions was revealed by BAER abnormalities in 7.4%. Thirty-five percent of the patients who had nystagmus and 53% of those who had internuclear ophthalmoplegia at the time of testing had BAER abnormalities. Forty-five percent of the abnormalities were elicited with stimulation of one ear only, stressing the importance of monaural stimulation. Click rates faster than 10 per second did not reveal abnormalities undetected at slower rates. BAERs were normal by these criteria in patients with labyrinthine diseases and amyotrophic lateral sclerosis. Thus, the BAER in MS can (1) confirm the presence of central lesions in patients with suspected brainstem involvement, (2) document the presence of clinically unsuspected lesions, and (3) be followed over time to provide possible assistance in evaluating the effectiveness of therapeutic measures. The BAER is a useful tool in the diagnosis and management of MS.