Background: The aim of this study was to investigate whether therapeutic factors as identified by Yalom and potential additional therapeutic factors could be found in the qualitative individual reports of high-risk adolescents with personality disorders at the end of an intensive group psychotherapeutic MBT programme and whether the therapeutic factors were related to therapy outcomes. Methods: At the end of treatment, 70 adolescents were asked to write a farewell letter. Content analysis of the letters was performed by two independent raters, using the 12 therapeutic factors of Yalom and potential additional therapeutic factors as coding categories. The factors were related to outcome, operationalized as a decrease in psychological symptoms as measured with the Symptom Check List 90 (SCL-90). Results: All therapeutic factors of Yalom and four new factors were identified in the letters, ranging from 1 to 97%. The factors of 'cohesion' (97%), 'interpersonal learning output' (94%), 'guidance' (98%) and 'identification' (94%) were found in most letters. By contrast, 'universality' (1%), 'family re-enactment' (3%) and 'instillation of hope' (1%) were found in very few letters. The factors 'interpersonal learning input', 'self-esteem' and 'turning point' were significantly associated with therapeutic recovery. Conclusions: Large presence differences were encountered in therapeutic factors associated with resilience processes and the resolution of psychological distress. Although a relationship was found between certain factors and change in symptoms, it was unclear whether the factors had led to such change. Further research seems important for treatment in general and for the personalization of treatment.