The main aim of this study was to explore the experiences of interdisciplinary collaboration among staff members, applying cognitive milieu therapy for inpatients with dual diagnosis. The study is based on 33 individual, qualitative interviews. The interdisciplinary approach helped the staff to agree on common aims for the treatment of each patient. The cognitive method was useful and provided the staff with suitable tools in their work with the patients. The staff succeeded in developing a good learning climate, where they cooperated on equal terms with the patients. Collaborative team learning and external supervision facilitated personal learning processes.