This study prospectively examined the effects of adolescent (N=81) and professional caregiver (N=33) attachment representations, measured using the Adult Attachment Interview, on therapeutic relationships in a youth treatment institution. After the first 3 months of the clients' stay in the institution, no effects of adolescents' or mentors' (i.e., the professional caregiver assigned) security and type of attachment representations were found. In a subgroup of 28 clients staying for a longer period, more secure adolescents were perceived by their mentors as increasing their secure base use and decreasing avoidance of contact, whereas more secure mentors were increasingly perceived as available as a secure base. Moreover, specific combinations of attachment representations of mentor and adolescent had different effects on adolescent hostility. Copyright 2006 by the American Psychological Association.