This study aimed to evaluate the effectiveness of positive systemic practice (PSP) in the context of a controlled trial. PSP is a model of family therapy for adolescent emotional and behavioural problems. A series of thirty‐seven consecutive patients assigned to a PSP treatment group and thirty‐five similar patients consecutively assigned to a waiting‐list control group completed this trial. They were assessed at baseline (Time 1) and 16 weeks later (Time 2) using the strengths and difficulties questionnaire (SDQ) that evaluates adolescent emotional and behavioural problems, and the systemic clinical outcomes and routine evaluation (SCORE) that evaluates family functioning. The PSP group was also followed up 6 months later (Time 3). For trial completers, the clinical improvement rate on the SDQ in the treatment group (65%) was significantly greater than that of the control group (20%) at Time 2 and the improvement was maintained at Time 3 (60%). The amelioration in adolescent behaviour problems on the SDQ was correlated with improvement in family adjustment on the SCORE. Treatment completers reported a high level of treatment satisfaction with this model of family therapy. Practitioner points PSP is a promising model of family therapy for adolescent emotional and behavioural problems. A distinctive feature of the model is the provision, by a co‐therapy team, of conjoint family sessions and parallel series of sessions to adolescents and parents. The model is described in a treatment manual.