PRIMARY OBJECTIVE: To examine aggression in adolescent males with and without traumatic brain injury (TBI). DESIGN: Quasi-experimental, matched-participants design. METHODS: Thirty-nine adolescents were examined, including a TBI group (n = 11) and a matched, uninjured comparison group (n = 28). Participants with TBI were injured an average of 8.3 years (SD = 4.2 years) prior. Participants and parents completed the Youth Self Report (YSR) and the Child Behavior Checklist (CBCL), both measures of global psychopathology. In addition, the theoretically-driven Reactive and Proactive Aggression Questionnaire (RPQ) and the Form of Aggression Scale (FAS) were completed. MAIN OUTCOMES AND RESULTS: When compared using the YSR and CBCL, no group differences were detected. In contrast, using the self-report RPQ, participants with TBI reported more reactive and proactive aggression than non-injured peers. Using the FAS, participants with TBI reported engaging in aggression out of frustration and not to dominate or acquire objects. CONCLUSION: Aggressive behaviours are a long-term outcome after paediatric TBI. Measures of global psychopathology do not permit detailed examination of specific behaviour problems such as aggression, which may provide inaccurate data from which to derive incidence rates. In contrast, theoretically-driven measures can provide greater insight into post-TBI aggression with important treatment implications.