Major trauma is commonly defined using an Injury Severity Score (ISS) threshold of 15. Since this threshold was formulated, there have been significant developments in both the Abbreviated Injury Scale underlying the ISS, and trauma management techniques, both in the preventive and acute-care phases of trauma management. This study assesses whether this ISS threshold is appropriate when evaluating both mortality, and hospital-based indicators of morbidity, in a paediatric population using a large hospital trauma registry. Other registries and datasets using ISS >15 as an inclusion criterion may exclude a substantial body of data relating to significantly morbid trauma patients.