Intracranial compliance, as estimated from a computerized frequency analysis of the intracranial pressure (ICP) waveform, was continuously monitored during the acute postinjury phase in 55 head-injured patients. In previous studies, the high-frequency centroid (HFC), which was defined as the power-weighted average frequency within the 4- to 15-Hz band of the ICP power density spectrum, was found to inversely correlate with the pressure-volume index (PVI). An HFC of 6.5 to 7.0 Hz was normal, while an increase in the HFC to 9.0 Hz coincided with a reduction in the PVI to 13 ml and indicated exhaustion of intracranial volume-buffering capacity. The mean HFC for individual patients in the present study ranged from 6.8 to 9.0 Hz, and the length of time that the HFC was greater than 9.0 Hz ranged from 0 to 104.8 hours. The mortality rate increased concomitantly with the mean HFC, from 7% when the mean HFC was less than 7.5 Hz to 46% when the mean HFC was 8.5 Hz or greater. The length of time that the HFC was 9.0 Hz or greater was also associated with an increased mortality rate, which ranged from 16% if the HFC was never above 9.0 Hz to 60% if the HFC was 9.0 Hz or greater for more than 12 hours. In 12 patients who developed uncontrollable intracranial hypertension or clinical signs of tentorial herniation during the monitoring period, 75% were observed to have had an increase in the HFC to 9.0 Hz or more 1 to 36 hours prior to the clinical decompensation. The more rapid the increase in the HFC, the more likely the deterioration was to be caused by an intracranial hematoma. Continuous monitoring of intracranial compliance by computerized analysis of the ICP waveform may provide an earlier warning of neurological decompensation than ICP per se and, unlike PVI, does not require volumetric manipulation of intracranial volume.