ObjectiveTo study the relationship between arterial blood pressure, intracranial pressure, directly measured brain tissue oxygenation (Ptio2), and middle cerebral artery blood flow velocity in severely head-injured patients.DesignProspective study.SettingNeurosurgical intensive care unit.PatientsA total of 14 patients with severe head injury.InterventionsPharmacologic blood pressure manipulations using norepinephrine.Measurements and Main ResultsWe assessed the magnitude of Ptio2related to changes in cerebral perfusion pressure in 12 of the patients. We calculated in all the static rate of regulation, which is an index to describe the change of cerebrovascular resistance, using cerebral artery blood flow velocity in relation to changing cerebral perfusion pressure. Finally, we calculated the rate of change in Ptio2, which quantifies the percentage of change in Ptio2divided by the percentage of change in cerebral perfusion pressure. It is a new marker for cerebral tissue oxygen regulation based on direct measurement of Ptio2. There was a plateau phase for the cerebral perfusion pressure–Ptio2relation that was similar to the autoregulatory plateau seen in the relationship between cerebral perfusion pressure and cerebral artery blood flow velocity. The rate of change in Ptio2demonstrated a significant correlation with the static rate of regulation (R= −.61,p< .05). A decrease in intracranial pressure when arterial blood pressure increased from 70 to 90 mm Hg was strongly correlated with static rate of regulation (R= .79,p< .001).ConclusionsCerebral tissue Po2demonstrates a plateau phase similar to what is known about cerebral blood flow velocity, which suggests a close link between cerebral blood flow and oxygenation. Static cerebral autoregulation is significantly correlated with cerebral tissue oxygen reactivity.