Objective: To examine impulsivity changes after a severe traumatic brain injury (TBI) and to explore the relationships between impulsivity dimensions (urgency, lack of premeditation, lack of perseverance, sensation seeking), emotional/behavioural hyperactivation and 12-month outcome. Methods: Measures of emotional/behavioural hyperactivation and functional outcome were administered to 60 patients with severe TBI 12 months after the trauma. A scale designed to assess impulsivity changes after TBI was completed by the patients' significant others at the same time. Results: Scores on urgency and lack of perseverance were higher after the trauma, whereas the score on sensation seeking was lower and the score on lack of premeditation remained stable. Urgency was the only dimension of impulsivity related to both emotional/behavioural hyperactivation and functional outcome. The relationship between urgency and functional outcome was mediated by emotional/behavioural hyperactivation, suggesting that a high level of urgency results in emotional/behavioural hyperactivation, which in turn impacts functional outcome. Lack of perseverance was significantly associated with functional outcome, indicating that the higher the lack of perseverance, the lower the functional outcome. Conclusion: The results contribute to a better understanding of the 12-month outcome in patients with severe TBI. They also open interesting perspectives on management strategies for implementing targeted psychological interventions to decrease impulsive manifestations.