OBJECTIVE: To develop a validated mortality prediction score for children with traumatic injuries. PATIENTS AND METHODS: We identified all children (<18 years of age) in the US military established Joint Theater Trauma Registry from 2002 to 2009 who were admitted to combat-support hospitals with traumatic injuries in Iraq and Afghanistan. We identified factors associated with mortality using univariate and then multivariate regression modeling. The developed mortality prediction score was then validated on a data set of pediatric patients (≤18 years of age) from the German Trauma Registry, 2002–2007. RESULTS: Admission base deficit, international normalized ratio, and Glasgow Coma Scale were independently associated with mortality in 707 patients from the derivation set and 1101 patients in the validation set. These variables were combined into the pediatric “BIG” score (base deficit + [2.5 × international normalized ratio] + [15 − Glasgow Coma Scale), which were each calculated to have an area under the curve of 0.89 (95% confidence interval: 0.83–0.95) and 0.89 (95% confidence interval: 0.87–0.92) on the derivation and validation sets, respectively. CONCLUSIONS: The pediatric trauma BIG score is a simple method that can be performed rapidly on admission to evaluate severity of illness and predict mortality in children with traumatic injuries. The score has been shown to be accurate in both penetrating-injury and blunt-injury populations and may have significant utility in comparing severity of injury in future pediatric trauma research and quality-assurance studies. In addition, this score may be used to determine inclusion criteria on admission for prospective studies when accurately estimating the mortality for sample size calculation is required.