Abstract: Adolescent eating disorder symptoms, depression and anxiety, the impact of their symptoms on their parents, and parental self-efficacy were assessed before beginning family-based day hospital treatment, and at 3 and 6 months post-assessment. Parents’ self-efficacy increased during the first 3 months of treatment, and their knowledge and confidence in their effectiveness against the eating disorder continued to increase between 3 and 6 months post-assessment. Adolescent eating disorder symptoms, depression and anxiety, and the impact of the symptoms on their parents decreased between 3 and 6 months post-assessment. The results suggest that family-based treatment can be adapted to day hospital programmes for adolescents. The results also provide preliminary support for a treatment duration of at least 6 months.