BACKGROUND: Approximately 20 % of children and adolescents who have sustained mild traumatic brain injuries may experience long-term consequences, including cognitive problems, post-traumatic stress symptoms and reduced load-bearing capacity. The underestimation and belated recognition of these long-term consequences may lead to chronic and disruptive problems, such as participation problems in school and in social relationships. The aim of this study is to examine the level of activities and participation of children and adolescents up to 6 months after a mild traumatic brain injury and to identify possible outcome predictors. Another aim is to investigate the effectiveness of an early psychoeducational intervention and compare the results with those obtained with usual care. METHODS/DESIGN: This paper presents the Brains Ahead! study design, a randomised controlled trial nested within a multicentre, longitudinal, prospective cohort study. The eligible participants include children and adolescents between 6 and 18 years of age who have experienced a mild traumatic brain injury within the last 2 weeks. The cohort study will include 500 children and adolescents with a mild traumatic brain injury and their caregivers. A subset of 140 participants and their caregivers will be included in the randomised controlled trial. Participants in the randomised controlled trial will be randomly assigned to either the psychoeducational intervention group or the usual care control group. The psychoeducational intervention involves one face-to-face contact and one phone contact with the interventionist, during which the consequences of mild traumatic brain injury and advice for coping with these consequences to prevent long-term problems will be discussed. Information will be provided both verbally and in a booklet. The primary outcome domain is activities and participation, which will be evaluated using the Child and Adolescent Scale of Participation. Participants are evaluated 2 weeks, 3 months and 6 months after the mild traumatic brain injury. DISCUSSION: The results of this study will provide insight into which children with mild traumatic brain injury are at risk for long-term participation problems and may benefit from a psychoeducational intervention. TRIAL REGISTRATION: Netherlands Trial Register identifier NTR5153 . Registered on 17 Apr 2015.