Background: A need-supportive group climate is a prerequisite for successful treatment in secure residential youth care. For girls, positive relationships with peers are an essential part of the group climate. Relational aggression threatens the residential group climate. Objectives: This study explored whether a group counseling program can actively promote the quality of the interrelationships between girls in a residential group in the Netherlands; the aim is to elucidate how residential group climate affects treatment results. The girls in this single-gender residential group had been admitted because of (suspicions of) commercial sexual exploitation. Method: The study combined participatory observations with a case-based time-series design. For 18 weeks, changes in the residential group climate and the interrelationships between the girls and achievement of treatment goals over time were measured. Results: The residential group climate in this group was negative initially; much relational aggression between girls was found. Considering treatment, some girls were making significant progress, but others mainly deteriorated. Moderate improvement of residential group climate was visible after a group social worker was permanently present. Conclusions: A group counseling program aimed at improving interrelationships cannot cure an overall negative residential group climate. This article offers implications for preventing and recognizing deterioration of residential group climate and improving girls' residential group climate.