Background
Evidence-based outpatient psychotherapeutic programs are first-line treatment of borderline personality disorder (BPD). Early and effective treatment of BPD is crucial to the prevention of its individual, psychosocial, and economic consequences. However, in spite of recent advantages in diagnosing adolescent BPD, there is a lack of cost-effective evidence-based treatment programs for adolescents. Mentalization-based treatment is an evidence-based program for BPD, originally developed for adults.

Methods/Design
Aims/hypotheses: We will investigate whether a specifically designed mentalization-based treatment in groups is an efficacious treatment for adolescents with BPD or subthreshold BPD compared to treatment as usual. The trial is a four-center, two-armed, parallel-group, assessor-blinded randomized clinical superiority trial. One hundred twelve patients aged 14 to 17 referred to Child and Adolescent Psychiatric Clinics in Region Zealand are randomized to 1 year of either mentalization-based treatment in groups or treatment as usual. Patients will be included if they meet at least four DSM-5 criteria for BPD. The primary outcome is self-reported borderline features at discharge. Secondary outcomes will include self-harm, depression, BPD criteria, externalizing and internalizing symptoms, and social functioning, together with parental reports on borderline features, externalizing and internalizing symptoms. Measures of attachment and mentalization will be included as mediational
variables. Follow-up assessment will take place at 3 and 12 months after end of treatment.

Discussion
This is the first randomized controlled trial to test the efficacy of a group-based mentalization-based treatment for adolescents with BPD or subthreshold BPD. If the results confirm our hypothesis, this trial will add to the treatment options of cost-effective treatment of adolescent BPD.

Trial registration
Clinicaltrials.gov NCT02068326, February 19, 2014.