A new concept of cerebral hemodynamic and metabolic physiology, cerebral hemodynamic reserve (CHR), was evaluated in 20 comatose adults with acute traumatic brain swelling who were undergoing continuous monitoring of the arteriojugular difference in oxyhemoglobin saturation, along with cerebral perfusion pressure and expired PCO2. The CHR was measured as the ratio of relative (percent) changes in cerebral oxygen extraction to relative changes in cerebral perfusion pressure during spontaneous increases in intracranial pressure. In patients with initially severe brain swelling, the CHR was more frequently abnormal (compromised) on the first day, and tended to improve on the second day. In patients with initially moderate brain swelling, the CHR was more frequently normal (preserved) on the first day, but tended to become compromised on the second day if the brain swelling exacerbated. It is concluded that cerebral hemodynamic reserve abnormalities very closely associate with signs of increased intracranial "tightness" on computed tomographic scans of the head. Cerebral hemodynamic reserve could therefore become an important guide in the functional evaluation and management of acute brain swelling (focusing on cerebral oxygenation and perfusion pressure) in a variety of predominantly diffuse acute intracranial disorders.