Background Behavioural problems are common among adolescents. The burden on society in social disturbance, health, failures to contribute and costs has triggered innovative community‐based interventions such as multisystemic therapy (MST). Aims Our aim was to compare the cost‐effectiveness of MST and treatment as usual (TAU). Methods Cost‐effectiveness was assessed alongside a randomised clinical trial. One hundred and sixteen adolescents were allocated to MST or TAU. Follow‐up lasted six months. Quality of life (EQ‐5D) as perceived by the adolescents was the primary outcome. A societal perspective was used for cost assessment. Results There was no significant difference between groups in the small improvement experienced in quality of life (EQ‐5D average score improvement in both 0.02 points, standard deviation 0.13 MST; 0.23 TAU). Dropout before follow‐up was 48% and 69% respectively. Overall costs attributed to these young people were, however, 50% lower in the MST group. The incremental cost‐effectiveness ratio (ICER) was 384,633€ (95% CI: −2,001,433 to 2,024,681€), which indicates dominance of MST over TAU. Conclusions/Implications for practice Our study shows that, at worst, MST offers no advantage to young offenders in terms of their experienced quality of life, but ‘TAU’ included family focused intervention as well as standard supervision. There were some cost advantages for the individual and his/her family in the MST group, but substantial cost benefits for wider society. The case for a large, multi‐centre, perhaps international trial is strong as widespread implementation of MST would benefit everyone if these findings are confirmed. Copyright © 2016 John Wiley & Sons, Ltd.