Traumatic coagulopathy manifests as a hypocoagulable state associated with hypothermia, acidosis, and coagulation factor dilution. The diagnosis must be made clinically because traditional coagulation tests are neither sensitive nor specific and take too long to be used for intraoperative monitoring. We hypothesized that the activated coagulation time (ACT) would reflect the global coagulation status of traumatized patients and would become elevated as coagulation reserves become exhausted. A prospective protocol was used to study 31 victims of major trauma who underwent immediate surgical intervention. Victims of major head trauma were excluded and patients were selected at random over an 8-month period. At least two serial intraoperative blood samples were obtained at 15-min intervals via indwelling arterial catheters. A Hemochron model 801 coagulation monitor was used to measure the ACT. Of the 31 patients studied, 7 became clinically coagulopathic and 24 did not. The ACT measurements of coagulopathic and noncoagulopathic trauma patients were significantly different by multiple statistical comparisons. Both groups differed from normal, nontraumatized patients. The coagulopathic trauma patients had significantly elevated values when compared with other trauma patients or to normal values. We conclude that a low ACT reflects the initial hypercoagulability associated with major trauma and an elevated ACT is an objective indicator that the coagulation system reserve is near exhaustion. An elevated ACT may represent an indication for considering damage control maneuvers or more aggressive resuscitation.