Disturbance of cerebral haemodynamic status, cerebral vasoreactivity (CVR) to carbon dioxide (CO2) and pressure autoregulatory response (PAR) have been shown to be associated with severity of head injury and to correlate with neurological condition and clinical outcome. Information regarding impaired CVR and/or lost PAR has been shown to be of prognostic value in treating head-injured patients. The classic method of evaluating cerebral haemodynamic status requires an invasive measurement of cerebral blood flow (CBF) such as the technique of Kety and Schmidt or by the 133Xe inhalation method. Transcranial Doppler ultrasonography (TCD) is a simple and non-invasive method for its assessment. In this study, cerebral haemodynamic status as determined by TCD was compared with that of stable xenon-enhanced cerebral blood flow (XeCBF) which is generally regarded as a gold standard in CBF measurement with minimal complication, suggesting that employing CO2 and blood pressure challenge with TCD measurement to assess the CVR and PAR in head-injured patients, provides a relatively less invasive method and can be repeated at least daily to assess the cerebrovascular autoregulatory reserve.