Acceptance and Commitment Therapy (ACT), which derives from CBT, has shown promising results in children with chronic functional pain. Improvement could be demonstrated by less avoidance of important activities, better emotional wellbeing and less health care utilization. The aim of this project is to develop an ACT-based group intervention for adolescents with a range of FSS, i.e. conceptualised as severe BDS, and to evaluate its efficacy in a randomized controlled trial.