Background: About 5 – 15 % of patients with concussion experience persistent post-concussion symptoms (PCS) longer than 3 months post-injury. Currently, treatment options are limited, and no evidence-based intervention is available. Recent studies suggest that cognitive and behavioral processes may be involved in symptom maintenance. Aim: To explore in an uncontrolled study design: 1) the overall outcome of a newly developed, brief behavioral intervention in young patients (15 – 30 years) with persistent PCS 3 – 6 months after concussion, and 2) the hypothesised mediators, i.e. illness perceptions and illness behaviors. Methods: Thirty-two patients, consecutively recruited from an epidemiological study or referred by primary care physicians, received an 8-week interdisciplinary intervention carried out by a physiotherapist, an occupational therapist and a neuropsychologist. The intervention was based on principles from cognitive-behavioral therapy and gradual return to activities. The primary treatment objective was to reduce PCS and prevent chronification of symptoms by reducing negative illness perceptions (eg. beliefs that one cannot control the symptoms and that they will persist), and reducing maladaptive illness behaviors (eg. excessive rest, exertion or oscillation between these opposites). Self-report measures were completed pre-intervention, post-intervention, and at 3 and 12 months post-intervention. Change after intervention was measured by the Rivermead Post-concussional Symptoms Questionnaire (RPQ) (primary outcome). Hypothesised mediators were measured by the Brief Illness Perception Questionnaire (B-IPQ) and the Behavioural Response to Illness Questionnaire (BRIQ). Patients’ experiences were evaluated by Experience of Service Questionnaire (ESQ). Data was analysed using an unadjusted mixed model. Results: Mean age was 23.3 years, and 81 % were women. Patients showed a reduction in PCS from 35.2 points pre-intervention to 25.6 points post-intervention (N=23, d=9.6 (95% CI 5.2 - 13.9), p< 0.001, ES = 1.04), as well as a reduction in negative illness perceptions (N=23, p<0.001) and maladaptive illness behaviors (N=23, p<0.05). Improvements were maintained at 3 months (N=20) and 12 months (N=26) follow-up. Furthermore, patients' responses to the ESQ post-intervention showed that 95% rated the intervention as good, and 100 % would recommend the intervention to a friend (N=23). Conclusion: Based on uncontrolled data, this new intervention was associated with significant symptom reduction as well as more adaptive illness perceptions and illness behaviors. Thus, it may have the potential to prevent chronification of PCS. In addition, the intervention was feasible and well received by the patients. An RCT is currently performed to evaluate the effect of the intervention and to further test the hypothesised mediators, i.e. illness perceptions and illness behaviour.