Mentalization-based treatment (MBT) is an evidence-based treatment for adults suffering from borderline personality disorder. Different adaptations of MBT for adolescents have been described, but almost none of these have been systematically evaluated so far. This article presents pilot data from a feasibility study of an adaptation of inpatient MBT for adolescents with borderline symptoms (MBT-A). Preliminary outcome results were examined in a pilot study including 11 female adolescents (aged 14-18 years) in a mental health care center in the Netherlands. Maximum treatment duration was 12 months and patients were assessed at start and at 12 months after start of treatment. Outcome measures included symptom severity (Brief Symptom Inventory), personality functioning (Severity Indices of Personality Problems), and quality of life (EuroQol). Results showed significant decreases in symptoms, and improvements in personality functioning and quality of life at 12 months after start of treatment. Effect sizes (d) ranged from .58 to 1.46, indicating medium to large effects. In total, 91% of the adolescents showed reliable change on the BSI, and 18% also moved to the functional range on the BSI. The results of this feasibility study are promising and encourage further research concerning the efficacy of MBT in adolescents with borderline symptoms, although some problems with implementation suggest that an outpatient variant of MBT for adolescents might be as effective while at the same time reducing potential iatrogenic effects of inpatient treatment for this age group.