This study was designed to evaluate the alterations in brainstem auditory evoked potentials (BAEPs) in patients with head injuries. BAEPs were recorded of twenty-five normal subjects to serve as control group. Fifty-four patients with head injuries were evaluate prospectively by neurological check-ups, the Glasgow coma rating scale, brain CT scans, as well as BAEPs. Among them, twenty-two patients accepted a follow-up evaluation one month after the first study. In the acute stage, though the significant prolongation of peak latency occurred only in the right peak III as compared with the control group, the rate of presence decreased significantly in most of the peak waves. There were three main types of abnormality: isoelectric BAEP (13.0%), alterations in peak I (29.6%) and in interpeak latencies (13.0%). The incidence of abnormal BAEPs was 48.2% of 54 patients. Patients were divided into two groups according to whether their coma scale was above/equal to or below 9. No significant difference was shown in the rate of appearance of wave peaks and the average latency of each peak wave. This suggests BAEPs displayed a function of the auditory pathway only rather than a reticular formation in the brainstem. Concerning the group with brain stem abnormality as shown in CT scan, there was significant prolongation in most peak and interpeak latencies, and the rate of presence of each peak was significantly lower compared with the non-brainstem-abnormality group. Therefore, the alteration in BAEPs in head injuries coincide with the presence of abnormal brainstem signs in the CT.(ABSTRACT TRUNCATED AT 250 WORDS)