The goal of this three-wave longitudinal study was to analyze foster parent stress and foster children’s internalizing and externalizing behaviors in a transactional framework. Participants in this study were 237 children in foster care in the Netherlands with, mostly, long placement durations (M = 56.86 months, SD = 49.10 months). We examined concurrent, prospective unidirectional and bidirectional relations between foster children’s behavior and foster parent stress by using cross-lagged structural equation modeling and examined whether the results were stable across different subgroups of foster children. In contrast to our hypothesis, we found no bidirectional relations. There were unidirectional prospective pathways from foster children’s internalizing and externalizing problems to foster parent stress, but no significant prospective pathways from foster parent stress to foster children’s internalizing and externalizing problems. The results were fairly stable across different subgroups of foster children. The lack of bidirectional relations was unexpected given the presence of transactional relations in biological parent-child dyads. Foster parents seem not to influence their foster children when it comes to regulating problem behavior. Therefore, the question is whether foster parents can, in more general terms, help their foster children benefit from their improved home environment.