The β‐amyloid peptides, Aβ1‐42 and Aβ1‐40, were quantified in ventricular CSF taken daily for up to 3 weeks from six individuals with severe traumatic brain injury (TBI). There was considerable interindividual variability in the levels of Aβ peptides, but in general Aβ1‐42 levels equalled or exceeded those of Aβ1‐40. Averaging the daily totals of our trauma cohort revealed that the levels of Aβ1‐42 and Aβ1‐40 rose after injury, peaking in the first week and then declining toward control levels over the next 2 weeks. Aβ1‐42 levels were on average two to three times higher in the trauma cohort than in CSF from nontrauma samples. Compared with nontrauma samples, the Aβ1‐40/Aβ1‐42 ratio decreased about fivefold in the trauma patients, further indicative of increased Aβ1‐42 levels. The ratio remained low at all time points studied. No change was measured in the levels of β‐amyloid precursor protein during the same interval. These results suggest that Aβ1‐42 becomes elevated in the CSF after severe brain trauma.