Background: Five – 15 % of patients with concussion experience impairing post-concussional symptoms (PCS) more than 3 months post-injury. Currently, treatment studies are scarce and no evidence-based treatment is available. Existing literature suggest that negative illness perceptions and maladaptive illness behaviours may be involved in the development of persisting PCS. Aim: To develop an early behavioural intervention for young patients (15 – 30 years) with continuing PCS 3-6 months post-concussion, to explore hypothesised mediators, i.e. illness perceptions and illness behaviour, and the overall effect of the intervention in a pilot study. Methods: An interdisciplinary intervention programme was developed based on principles from cognitive-behavioural therapy and graded exercise therapy. The primary treatment objective was to reduce PCS and prevent chronification of symptoms by improving negative illness perceptions and decreasing maladaptive illness behaviour. Thirty-two patients attended the pilot study and completed self-report measures before the intervention, post-intervention and 3 months post-intervention. Change after intervention was assessed by Rivermead Post-concussional Symptoms Questionnaire (RPQ) (primary outcome) and measures of emotional distress and quality of life. Hypothesised mediators were measured by the Brief Illness Perception Questionnaire (B-IPQ) and the Behavioural Response to Illness Questionnaire (BRIQ). Results: Primary out-come data were available for 29 patients. Mean age was 23.3 years, and 78 % were women. Based on paired t-test using last-observation carried forward to replace missing values, patients showed a reduction in PCS from 36.1 points before the intervention to 28.0 points post-intervention (p=0.0003). There was no significant change from post-intervention to follow-up 3 months post-intervention (0.5 points, p=0.76). Further details on the treatment rationale and on changes of the hypothesised mediators will be presented. Conclusion: Based on uncontrolled pilot data this new intervention programme was associated with a significant reduction in PCS. It may have the potential to prevent chronification of symptoms by improving negative illness perceptions and maladaptive illness behaviours. An RCT is currently performed to compare the effect of the intervention programme with the effect of enhanced usual care.