The transient hyperemic response test has been shown to provide an index of cerebral autoregulation in healthy individuals and in patients who have suffered a subarachnoid hemorrhage. In this study, the test was applied to patients who had received a severe head injury, and the value of the test was assessed by comparing its result with the individual's clinical condition (Glasgow Coma Scale [GCS] score), cerebral perfusion pressure (CPP), transcranial Doppler wave form-derived index for cerebral autoregulation (relationship between the CPP and the middle cerebral artery flow velocity), and outcome (Glasgow Outcome Scale [GOS] score). Forty-seven patients, aged 16 to 63 years, with head injuries were included in the study. Signals of intracranial pressure, arterial blood pressure, flow velocity, and cortical microcirculatory flux were digitized and recorded for a period of 30 minutes using special computer software. Two carotid compressions were performed at the beginning of each recording. The transient hyperemic response ratio (THRR: the ratio of the hyperemic flow velocity recorded after carotid release and the precompression baseline flow velocity) was calculated, as was the correlation coefficient Sx used to describe the relationship between slow fluctuations in the systolic flow velocity and CPP throughout the period of recording. No significant changes in CPP were found during compression. There was a significant correlation between the THRR and the Sx (r = 0.49, p < 0.0001). The hyperemic response proved to be lower in patients who exhibited a poor clinical grade at presentation (GCS scores < 6, p = 0.01) and lower in patients achieving a poor outcome (GOS scores of 3, 4, and 5, p = 0.003). Loss of postcompression hyperemia occurred when the CPP fell below 50 mm Hg. The carotid compression test provides a simple index of cerebral autoregulation that is relevant to the clinical condition and outcome of the severely head injured patient.