Objective - To examine the animal trauma triage (ATT) and modified Glasgow Coma Scale (mGCS) scores as predictors of mortality outcome (death or euthanasia) in injured dogs.Design - Observational cohort study conducted from September 2013 to March 2015 with follow-up until death or hospital discharge.Setting - Nine veterinary hospitals including private referral and veterinary teaching hospitals.Animals - Consecutive sample of 3,599 dogs with complete data entries recruited into the Veterinary Committee on Trauma patient registry.Interventions - None.Measurements and Main Results - We compared the predictive power (area under receiver operating characteristic [AUROC]) and calibration of the ATT and mGCS scores to their components. Overall mortality risk was 7.3% (n = 264). Incidence of head trauma was 9.5% (n = 341). The ATT score showed a linear relationship with mortality risk. Discriminatory performance of the ATT score was excellent with AUROC = 0.92 (95% confidence interval [CI] 0.91 to 0.94) and pseudo R-2 = 0.42. Each ATT score increase of 1 point was associated with an increase in mortality odds of 2.07 (95% CI = 1.94-2.21, P < 0.001). The "eye/muscle/integument" category of the ATT showed poor discrimination (AUROC = 0.55). When this component together with the skeletal and cardiac components were omitted from calculation of the overall score, there was no loss in discriminatory capacity (AUROC = 0.92 vs 0.91, P = 0.09) compared with the full score. The mGCS showed good performance overall, but performance improved when restricted to head trauma patients (AUROC = 0.84, 95% CI = 0.79-0.90, n = 341 vs 0.82, 95% CI = 0.79-0.85, n = 3599). The motor component of the mGCS showed the best predictive performance (AUROC = 0.79 vs 0.66/0.69); however, the full score performed better than the motor component alone (P = 0.002). When assessment was restricted to patients with head injury (n = 341), the ATT score still performed better than the mGCS (AUROC = 0.90 vs 0.84, P = 0.04).Conclusions - In external validation on a large, multicenter dataset, the ATT score showed excellent discrimination and calibration; however, a more parsimonious score calculated on only the perfusion, respiratory, and neurological categories showed equivalent performance.