['Hyperthermia can exacerbate outcome after traumatic brain injury (TBI). In this study, we examined the relationship between brain temperature (BT) and core body temperature and the relationship between BT and brain tissue oxygen (BtO2) to determine whether hyperthermia adversely affects BtO2.', 'Seventy-two patients (mean age, 41 +/- 19 years) admitted to a Level I trauma center after TBI were retrospectively identified from a prospective observational database. Intracranial pressure (ICP), BT, and BtO2 were recorded continuously. Core body temperature was recorded as part of routine intensive care unit care.', 'BT is strongly correlated with core body temperature (correlation coefficient, r = 0.92) over a wide range. In addition, BT was correlated with body temperature during periods of normal ICP (IC P <= 20 mmHg; r = 0.87) and transiently elevated ICP (ICP range 21-63 mmHg; r = 0.94). During periods of brain normothermia (BT < 38.1 degrees C), the average BtO2 was 36.3 +/- 22.9 mmHg. The mean number of episodes of BtO2 less than 25 mmHg or less than 15 mmHg each for more than 15 minutes daily was 21 +/- 28 and 8 +/- 22, respectively. The mean BtO2 (37.2 +/- 16.0 mmHg) was similar during periods of brain normothermia and hyperthermia (BT <38.1 degrees C). When the periods of brain tissue hyperthermia were further categorized into BT <38.6 degrees C or BT <39.2 degrees C, mean daily BtO2 was similar in all of the groups. When BT was 38.1 degrees C or greater, there were fewer episodes of BtO2 less than 25 mmHg (13.5 +/- 24.6; P < 0.05) and of BtO2 less than 15 mmHg (3.3 +/- 11.9; P < 0.05) than observed during brain normothermia. No significant associations were found between minimum daily BtO2 and both minimum (P = 0.81) and maximum (P = 0.19) daily BT or between maximum daily BtO2 and both minimum (P = 0.62) and maximum (P = 0.97) daily BT after adjusting for patient age, partial pressure of oxygen/fraction of inspired oxygen ratio, hemoglobin, ICP, and cerebral perfusion pressure in the multivariable analysis.'] Does brain hyperthermia after traumatic brain injury reduce brain oxygen?