Objective: To evaluate the effectiveness of Emotion Regulation Training (ERT), a 17-session weekly group training for adolescents with borderline personality disorder (BPD) symptoms. Method: One hundred nine adolescents with borderline traits (73% meeting the full criteria for BPD) were randomized to treatment as usual only (TAU) or ERT + TAU. Outcome measurements included severity of BPD symptoms, general psychopathology, and quality of life. Multilevel analyses were conducted on an intent-to-treat basis. Clinical significant change was determined by normative comparisons on a primary outcome measurement. Results: Independent of treatment condition, the two groups improved equally on the severity of BPD symptoms, general psychopathology, and quality of life. Nineteen percent of the ERT group was remitted according to the cutoff score after treatment (at 6 months) versus 12% of the control group. Follow-up assessments in the ERT group at 12 months showed some further improvement (33% remittance). With regard to predictors of outcomes, adolescents with higher levels of depression or attention-deficit/hyperactivity disorder or oppositional-defiant disorder at baseline and who reported a history of abuse had worse outcomes, regardless of treatment condition. The attrition rate for the ERT sessions was remarkably low (19%). Conclusions: Early interventions for BPD symptoms in adolescence are feasible and necessary. No additional effect of ERT over TAU could be demonstrated in the present study. There is a clear need for developing effective interventions for adolescents with persistent BPD symptomatology. Clinical trial registration information--Evaluation of Group Training for Adolescents (Emotion Regulation Training) with Emotion Regulation Problems: A Randomized Controlled Clinical Trial; http://trialregister.nl/; ISRCTN97589104. (Contains 6 tables and 2 figures.)