Personality disorders are complex mental health problems, associated with chronic dysfunction in several life domains. Adolescents suffer from these disorders as well. The present study is a naturalistic case study, investigating whether group schematherapy (GST) can be applied to adolescents with personality disorders or personality disorder traits. Four clinically referred patients were included and completed questionnaires on quality of life, symptoms of psychopathology, schema modes, early maladaptive schemas, and schema coping styles. Patients participated in weekly GST sessions complemented by weekly or 2-weekly individual sessions. The parents of the adolescents participated in a separate parent group. From pre- to post-treatment, results demonstrated improvements for some patients in quality of life and symptoms of psychopathology. Changes in a number of modes and schemas were observed in all patients from pre- to post-therapy. In addition to assessing changes from pre- to post-treatment, the current study investigated the temporal changes in modes during therapy as well. Results demonstrated that maladaptive modes decreased, whereas healthy modes increased for all patients across the course of therapy. The present study provides preliminary support for the applicability of GST for adolescents as well as the effectiveness of GST. It is a starting point for further research on this intervention.