Objective: Premature termination, or dropout, is a major concern in psychotherapy in general and an issue of particular importance in treatments for borderline personality disorder (BPD). Yet few studies investigating dropout from therapy in adolescent BPD populations exist. This study investigates reasons for dropping out from group-based mentalization-based treatment (MBT-G) for BPD or borderline features in an adolescent population. Method: Ten semi-structured interviews were performed with female adolescents who had dropped out from group-based MBT for BPD. The data were analyzed qualitatively using Systematic Text Condensation. Results: The results point to the existence of a subgroup of adolescent BPD patients who do not perceive the treatment sufficiently helpful or worthwhile, who experienced treatment as emotionally demanding, time-consuming and connected with unpleasant experiences. Positive reasons for early termination were reported in the form of experienced improvement in condition. Dropping out became understood as a process of weighing perceived benefits against perceived costs of staying in treatment. This understanding of dropout as a process implies the existence of a window of time where intervention to prevent dropout is possible, presupposing the detection of at-risk patients.