Background: In residential youth care, group care workers and teachers often serve as a mentor for individual adolescents. Although favorable mentoring relationships are associated with positive adolescent outcomes, few studies examined the role of mentoring in residential youth care. Objective: The present study aims to assess adolescents', care workers' and teachers' mentoring relationship needs in terms of their one-on-one conversations during residential care. Methods: We conducted structured interviews with eleven adolescents, ten group care workers and two teachers and content analysis to assess the data. Results: All respondents are rather satisfied with their conversations, which are often concerned with how the adolescent is doing. Adolescents mostly consider their family and home-situation as difficult topics, while care workers mostly consider sexuality as a difficult topic to talk about. Although 'improvement' with the youth is often the aim, most adolescents report that they do not (know if they) show changes because of these conversations. Moreover, only one of the twelve professionals thinks that it is his core task to achieve "behavior change" with the adolescents. According to the professionals, conversations often aim at building a good relationship, coaching, determining treatment goals, and gaining insight into the adolescent. Adolescents prefer a mentor who is calm, has respect, listens, and is reluctant in giving advice. Most professionals do not use a specific method and doubt whether they want to have conversations according to a manual or support tool. Conclusions: Despite being rather satisfied, adolescents and professionals indicate several points for improvement of one-on-one conversations.