Epidemiological evidence links severe or repeated traumatic brain injury (TBI) to the development of Alzheimer's disease (AD). Accumulation of amyloid precursor protein (APP) occurs with high frequency after TBI, particularly in injured axons, and APP may be cleaved to amyloid-β (Aβ) peptides playing key pathophysiological roles in AD. We used cerebral microdialysis (MD) to test the hypothesis that interstitial Aβ levels are altered following TBI and are related to the injury type, cerebral energy metabolism, age of the patient, and level of consciousness. In the present report, we evaluated 10 mechanically ventilated patients (7 male, 3 female, ages 18-76 years) with a severe TBI, who had intracranial pressure and MD monitoring. Each MD sample was analyzed for hourly routine energy metabolic biomarkers (MD-lactate, MD-pyruvate, MD-glucose, and MD-lactate/pyruvate ratio), cellular distress biomarkers (MD-glutamate, MD-glycerol), and MD-urea. The remaining MD samples were analyzed for Aβ1-40 (Aβ40; n=765 samples) and Aβ1-42 (Aβ42; n=765 samples) in pooled 2 h fractions up to 14 days post-injury, using the Luminex xMAP technique, allowing detection with high temporal resolution of the key Aβ peptides Aβ40 and Aβ42. Data are presented using medians and 25th and 75th percentiles. Both Aβ40 and Aβ42 were consistently higher in patients with predominately diffuse axonal injury compared with patients with focal TBI at days 1-6 post- injury, Aβ42 being significantly increased at 113-116 h post-injury (p