In this study we assess the clotting onset time (COT) in samples from a population of traumatic brain injury patients. The patients were randomized to standard treatment plus high dose antithrombin (AT group) or standard treatment alone (nonAT group), during the first 16 hours after hospital admission. Our aim was to study the two patient groups during the first 5 days after injury, to assess COT as a coagulation monitoring method compared to routine parameters (thrombin-antithrombin complex (TAT), D-dimer, and soluble fibrin), and to correlate COT to clinical parameters and outcome. Clotting onset time measurements are carried out using free oscillating rheometry, where the endpoint of coagulation onset is determined by a deviation from initial viscoelastic properties of an oscillating sample. Both patient groups initially showed hypercoagulation. In the AT group, a significant increase of COT (i.e., decrease in hypercoagulation), was already seen 16 hours after hospital admission, but not until day 3 in the non AT group. Routine coagulation tests were not able to discriminate AT patients from nonAT patients. Clotting onset time correlated significantly to soluble fibrin, D-dimer, TAT, and leukocyte count. Additionally, COT levels at hospital admission correlated to outcomes measured with the Glasgow Outcome Scale (GOS) after 3 months. These results indicate that COT may be a clinically relevant variable with prognostic value, able to monitor the degree of hypercoagulation over time.