Background: An adaptation of multisystemic therapy (MST) was piloted to find out whether it would yield better outcomes than standard MST in families where the adolescent not only shows antisocial or delinquent behaviour, but also has an intellectual disability.

Method: To establish the comparative effectiveness of MST‐ID (n = 55) versus standard MST (n = 73), treatment outcomes were compared at the end of treatment and at 6‐month follow‐up. Pre‐treatment differences were controlled for using the propensity score method.

Results: Multisystemic therapy‐ID resulted in reduced police contact and reduced rule breaking behaviour that lasted up to 6 months post‐treatment. Compared to standard MST, MST‐ID more frequently resulted in improvements in parenting skills, family relations, social support, involvement with pro‐social peers and sustained positive behavioural changes. At follow‐up, more adolescents who had received MST‐ID were still living at home.

Conclusions: These results support further development of and research into the MST‐ID adaptation.