Background: Foster care is a relatively common arrangement when parents are unable to meet the needs of their children. Specialized foster care is sometimes applied in cases when problems are more serious and complex. More knowledge is needed about the effects of such specialized foster care. Aims: To explore trajectories of change associated with specialized foster care in a treatment model with a relational and mentalization-based orientation, and to develop the understanding of foster children's and their foster parents' experiences of their relationship living in a treatment foster family. Methods: Children and young people between the ages of 5 and 20 years who received treatment within a specialized foster care model, Treatment By Foster care (TBF), participated in this study. Longitudinal data collected in a naturalistic setting were analyzed quantitatively. In Studies 1 and 2, the number of participants at baseline varied for different instruments between 76 -105. The Achenbach System of Empirically Based Assessment (ASEBA) was used to measure how psychiatric symptoms change from the perspectives of the foster children, the foster parents, and teachers. The Adaptive Behavior Assessment Systemsecond edition (ABAS-II) was used to measure adaptive functioning from the foster parents' perspective. Self-ratings by the children and young people of their emotional and social problems were measured with the Beck Youth Inventories of Emotional and Social Impairment (BYI). Data about experiences of the relationship between child and foster parent were collected through repeated individual short interviews/speeches with both children and their foster parents according to Five Minute Speech Sample (FMSS) (n = 14). Interviews/speeches were analyzed using Thematic Analysis (TA). Results: The ratings of foster parents and foster children differed. The analyses showed a significant reduction in psychiatric symptoms, emotional and social problems according to the self-ratings by the children and young people. According to foster parents and teachers, psychiatric symptoms did not decrease. The baseline ratings of adaptive functioning by foster parents showed that adaptive functioning was considerably below peers from the Swedish non-clinical norm group. Adaptive functioning did improve but not enough to approach or catch up with peers.Analysis of the interviews/speeches generated three main themes containing seven subthemes. Main themes were: No 'real' family, A co-created relationship, and Time. Participants related to a norm for what a 'real' family is and seemed to presuppose that the biological family is the 'real' family. A co-created relationship related to No 'real' family as an answer or a solution. The challenges in the foster family constellation could be overcome by a mutual ambition to build a relationship and by liking each other. Time appeared as a common theme and both as an opportunity and a threat to the rela-tionship. Despite the fact that no interview question concerned the duration of the relationship, ...