Introduction: The Brain Injury Guidelines (BIG) stratifies complicated mild traumatic brain injury (mTBI) patients into 3 groups to guide hospitalization, neurosurgical consultation and repeat head-CT. BIG-1 patients could be managed safely without neurosurgical consultation or transfer. Systematic transfer to neurotrauma centers provide few benefits to this subgroup leading to overtriage. Similarly, unnecessary clinical and radiological follow-ups utilize significant health-care resources. Objective: to validate the safety and efficacy of the BIG for complicated mTBIs. Methods: We performed a multicenter historical cohort study in 3 level-1 trauma centers in Quebec. Patients ≥16 years old assessed in the Emergency Department (ED) with complicated mTBI between 2014 and 2017 were included. Patients with penetrating trauma, cerebral aneurysm or tumor were excluded. Clinical, demographic and radiological data, BIG variables, TBI-related death and neurosurgical intervention were collected using a standardized form. A second reviewer assessed all ambiguous files. Descriptive statistics, over- and under-triage were calculated. Results: A total of 342 patients’ records were assessed. Mean age was 63 ± 20,7 and 236 (69 %) were male. Thirty-five patients were classified under BIG-1 (10.2%), 110 under BIG-2 (32.2%) and 197 under BIG-3 (57.6%). Twenty-six patients (7%) required neurosurgical intervention, all were BIG-3. 90% of TBI-related deaths occurred in BIG-3 and none were classified BIG-1. Among the 192 transfers (51%), 14 were classified under BIG-1 (7.3%) and should not have been transferred according to the guidelines and 50 under BIG-2 (26%). In addition, 40% of BIG-1 received a repeat head computed tomography, although not indicated. Similarly, 7 % of all patients had a neurosurgical consult even if not required. Projected implementation of BIG would lead to 47% of overtriage and 0.3% of undertriage. Conclusion: Our results suggest that the Brain Injury Guidelines could safely identify patients with negative outcomes and could lead to a safe and effective management of complicated mTBI. Applying these guidelines to our cohort could have resulted in significantly fewer repeat head CTs, neurosurgical consults and transfers to level 1 neurotrauma centers.