Introduction: The purposes of this study were (a) to investigate the oxidative and antioxidative status in paediatric patients with varying severity of traumatic brain injury (TBI) during the early post-traumatic period; (b) to correlate injury severity with the oxidative and antioxidative status of patients; and (c) to identify early predictors of outcome in patients with TBI. Methods: Fifty-two consecutive paediatric patients with isolated TBI and 31 age- and sex-matched healthy controls were enrolled. Patients were divided into two groups based on their Glasgow Coma Scale (GCS) scores recorded upon admission to the emergency department within the first 24 hours after injury. Twenty-three patients with moderate to severe TBI and 29 patients with mild TBI (GCS scores of >13) were included. The primary outcome variable was hospital mortality. Plasma total oxidant status (TOS), total antioxidant status (TAS), and the oxidative stress index (OSI) were assessed as predictors of early oxidative changes in serum using a novel automated measurement method. Results: Compared to patients with mild TBI, TOS and OSI values were markedly elevated in patients with moderate to severe TBI. However, TAS levels did not show significant changes in either group of patients. Both GCS scores and Revised Trauma Scores were negatively correlated with the TOS and OSI, but neither was significantly correlated with the TAS. The TOS and OSI were significantly higher in non-survivors than in survivors. However, there was no significant difference in TAS levels between survivors and non-survivors. Conclusion: Paediatric patients with isolated TBI are exposed to extensive oxidative stress, which varies with injury severity.