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Severe traumatic brain injury (TBI) may result in widespread damage to axons, termed diffuse axonal injury. Alzheimer's disease (AD) is characterised by synaptic and axonal degeneration together with senile plaques (SP). SP are mainly composed of aggregated β-amyloid (Aβ), which are peptides derived from the amyloid precursor protein (APP). Apart from TBI in itself being considered a risk factor for AD, severe head injury seems to initiate a cascade of molecular events that are also associated with AD. We have therefore analysed the 42 amino acid forms of Aβ (Aβ(1-42)) and two soluble forms of APP (α-sAPP and βsAPP) in ventricular cerebrospinal fluid (VCSF) and Aβ(1-42) in plasma from 28 patients in a serial samples 0-11 days after TBI. The levels of α-sAPP, β-sAPP and Aβ(1-42) were determined using ELISA assays. After TBI, there was a significant stepwise increase in VCSF-Aβ(1-42) up to 1173 % from day 0-1 to day 5-6 and in VCSF-β-sAPP up to 2033 % increase from day 0-1 to day 7-11. There was also a slight but significant increase of VCSF-β-sAPP from day 0-1 to day 5-6 and day 7-11. By contrast, the plasma- Aβ(1-42) level is unchanged after injury. The marked increase in VCSFAβ( 1-42) implies that increased Aβ expression may occur as a secondary phenomenon after TBI with axonal damage. The unchanged level of plasma-Aβ(1-42) in contrast to the marked increase in VCSF-Aβ(1-42) after severe TBI, supports the suggestion that plasma Aβ(1-42) does not reflect Aβ metabolism in the central nervous system (CNS).