Objective: We examined whether mentalization-based treatment for adolescents (MBT-A) is more effective than treatment as usual (TAU) for adolescents who self-harm. Method: A total of 80 adolescents (85% female) consecutively presenting to mental health services with self-harm and comorbid depression were randomly allocated to either MBT-A or TAU. Adolescents were assessed for self-harm, risk-taking and mood at baseline and at 3-monthly intervals until 12 months. Their attachment style, mentalization ability and borderline personality disorder (BPD) features were also assessed at baseline and at the end of the 12-month treatment. Results: MBT-A was more effective than TAU in reducing self-harm and depression. This superiority was explained by improved mentalization and reduced attachment avoidance and reflected improvement in emergent BPD symptoms and traits. Conclusions: MBT-A may be an effective intervention to reduce self-harm in adolescents. Clinical trial registration information: The emergence of personality disorder traits in adolescents who deliberately self harm and the potential for using a mentalisation based treatment approach as an early intervention for such individuals: a randomised controlled trial; http://www.controlled-trials.com; ISRCTN95266816. (Contains 5 tables and 3 figures.)