Knowledge about the development of mental health in young people in foster care is limited. This naturalistic study examined the effects of a relational and mentalization-focused treatment in foster families in Sweden on the placed young people's mental health. The Achenbach System of Empirically Based Assessment (ASEBA) was used to measure change in psychiatric symptoms. Self-ratings showed significant improvements and medium to strong effects after 24 months in both boys and girls. No significant changes were found in the foster parents' ratings or in the school staff's ratings. Foster parents' ratings suggested that girls' behavioral problems decreased, but not the boys'. Based on these findings, we want to emphasize the importance of evaluating treatment effects using self-ratings by the young people in addition to parents' and parent substitutes' ratings.