Acute mountain sickness (AMS) is a common condition occurring within hours of rapid exposure to high altitude. Despite its frequent occurrence, the pathophysiological mechanisms that underlie the condition remain poorly understood. We investigated the role of cerebral oxygen metabolism (CMR(O(2))) in AMS. The purpose of this study was to test 1) if CMR(O(2)) changes in response to hypoxia, and 2) if there is a difference in how individuals adapt to oxygen metabolic changes that may determine who develops AMS and who does not. Twenty-six normal human subjects were recruited into two groups based on Lake Louise AMS score (LLS): those with no AMS (LLS ≤ 2), and those with unambiguous AMS (LLS ≥ 5). [Subjects with intermediate scores (LLS 3-4) were not included.] CMR(O(2)) was calculated from cerebral blood flow and arterial-venous difference in O(2) content. Cerebral blood flow was measured using arterial spin labeling MRI; venous O(2) saturation was calculated from the MRI of transverse relaxation in the superior sagittal sinus. Arterial O(2) saturation was measured via pulse oximeter. Measurements were made during normoxia and after 2-day high-altitude exposure at 3,800 m. In all subjects, CMR(O(2)) increased with sustained high-altitude hypoxia [1.54 (0.37) to 1.82 (0.49) μmol·g(-1)·min(-1), n = 26, P = 0.045]. There was no significant difference in CMR(O(2)) between AMS and no-AMS groups. End-tidal Pco(2) was significantly reduced during hypoxia. Low arterial Pco(2) is known to increase neural excitability, and we hypothesize that the low arterial Pco(2) resulting from ventilatory acclimatization causes the observed increase in CMR(O(2)).