Previous studies of the neurobiology of autism that have used the brainstem auditory evoked response have given contradictory results. The authors of this study considered two supplementary aspects; they added an ipsilateral masking procedure, and they compared the results for every subject to the values (corrected for age and sex) of a large number of normal children. Twenty autistic (according to DSM-III-R criteria) and 13 mentally retarded (nonverbal IQ less than 75) subjects were assessed. Eighty percent of the autistic subjects had abnormal interpeak latencies, compared to 15% of the mentally retarded subjects. The I-V and III-V prolonged interpeak latency values were seen only in the autistic subjects. The ipsilateral masking procedure doubled the rate of detection of higher-brainstem abnormalities in the autistic children.