Objective: To examine changes in personality disorders and symptomology and the relation between personality disorder variables and treatment outcomes in an adolescent sample during partial residential mentalization based treatment. Methods: In a sample of 62 (out of 115) adolescents treated for personality disorders, assessment was done pre- and post-treatment using the Structured Clinical Interview for DSM personality disorders and the Symptom Check List 90. Results: Significant reductions in personality disorder traits (t = 8.36, p = .000) and symptoms (t = 5.95, p = .000) were found. During pre-treatment, 91.8% (n = 56) of the patients had one or more personality disorders, compared to 35.4% (n = 22) at post-treatment. Symptom reduction was not related to pre-treatment personality disorder variables. Conclusion: During intensive psychotherapy, personality disorders and symptoms may diminish. Future studies should evaluate whether the outcomes obtained are the result of the treatment given or other factors.