OBJECTIVE To determine whether chronic conductive hearing loss in adults results in changes in the auditory brainstem response (ABR) similar to those observed in children with histories of otitis media with effusion. METHODS Test of effect of unilateral conductive hearing loss on adult ABR using age-matched control group and subjects as their own controls. METHODS Twelve adults with a history of unilateral conductive ear disease. An age-matched control group of 21 adults was also tested. METHODS The ABR, an electrophysiologic test of auditory brainstem functioning, was used to evaluate possible brainstem abnormalities in the impaired listeners. In addition, the masking-level difference, a behavioral test of binaural auditory processing in the brainstem, was used. RESULTS When comparing the patients' diseased ears with their healthy ears, significant delays were seen for wave V as well as for the I-V and III-V interwave intervals. For comparison with the control population, significant prolongations were again seen for wave V and for the III-V interwave intervals. In addition, reduced masking-level differences and significant correlations between the masking-level differences and the ABRs, independent of hearing threshold, were noted. CONCLUSIONS The results suggest that chronic conductive impairment in adults leads to changes in the ABR similar to those observed in children with histories of otitis media with effusion. As such, these changes do not appear to be related to a critical period of development.