Clinical studies investigating adolescents’ adherence to medicinal and non-pharmacological treatment in hospitalized psychiatric populations are few. In our study of 13–17 year old adolescents admitted to psychiatric inpatient care, the patients’ adherence to treatment in general, and to medicinal and non-pharmacological treatment individually, was examined, with special focus on family and clinical factors. Data on adherence was collected from patients’ hospital records. A step-wise logistic regression analysis was used to find the significant predictors of adherence to treatment. Good family or social network relational functioning and use of special services at school were positively related to adherence in adolescents. Involuntary treatment and self-mutilative behavior were related to non-adherence to treatment. The results indicate the importance of addressing adherence to treatment in adolescent psychiatric inpatients. More studies with larger samples are needed to investigate this area further.