Lactate has been regarded as a waste product of anaerobic metabolism of glucose. Evidence also suggests, however, that the brain may use lactate as an alternative fuel. Our aim was to determine the extent of lactate uptake from the circulation into the brain after traumatic brain injury (TBI) and to compare it with levels of lactate in the brain extracellular fluid. We recruited 19 patients with diffuse TBI, monitored with cerebral microdialysis and jugular bulb catheters. Serial arteriovenous (AV) concentration differences of glucose and lactate were calculated from arterial and jugular blood samples, providing a measure of net uptake or export by the brain. Microdialysis was used to measure brain extracellular glucose and lactate. In 17/19 patients studied for 5 days post-injury, there were periods of net lactate uptake into the brain, most frequently on day 3 after injury. Brain microdialysate lactate had a median (interquartile range [IQR]) concentration of 2.5 (1.5-3.2) mmol/L during lactate uptake and 2.2 (1.7-3.0) mmol/L during lactate export. Lactate uptake into the brain occurred at a median (IQR) arterial lactate concentration of 1.6 (1.0-2.2) mmol/L. Lactate uptake was associated with significantly higher AV difference in glucose values with a median (IQR) of 0.4 (0.03-0.7) mmol/L during uptake and 0.1 (-0.2-0.3) mmol/L during lactate export (Mann-Whitney U p=0.003). Despite relatively high brain lactate compared with arterial lactate concentrations, the brain appears to up-regulate lactate transport into the brain after TBI. This may serve to satisfy greater demands for energy substrate from the brain after TBI.