Correlations between changes in wave V of the auditory brain-stem response, intracranial pressure, and neurological signs of uncal herniation as graded by Plum and Posner were investigated in 12 patients with supratentorial mass lesion. In three of the five patients who later showed signs of uncal herniation, the latency of wave V, which presumably originated in the inferior colliculus, was already significantly prolonged. In 6 cases which progressed to the early third nerve stage of uncal herniation, the latency of wave V was prolonged in 5 cases. In 7 cases which progressed to the late third nerve stage or worse, the latency was prolonged in 6 cases and suppressed in one. In four of six attempts (three cases), lowering of the intracranial pressure by 10% glycerol solution resulted in normalization of the latency of wave V without clinical improvement. These results strongly indicate that measurement of wave V can predict the beginning of uncal herniation and can assess the effectiveness of medical or surgical decompression on brain-stem function.