BackgroundChildren in foster care tend to exhibit adverse psychosocial functioning, and foster parents tend to experience high levels of stress related to their role as carers.MethodsThe study included 60 foster children and 42 children living in biological families as a comparison group. Caregiver stress was measured using the Parenting Stress Index, while child problem behavior was measured using the Strengths and Difficulties Questionnaire. Children and their primary carer were assessed when the children were 2, 3, and 8 years, respectively.ResultsThe results showed higher stress scores for foster parents at all time points, with the difference being most pronounced in stress related to the child. Problem behavior was also higher for foster children at age 8 years. Foster parent stress increased in all domains during the course of the study. Child domain stress was the only significant predictor of child problem behavior.ConclusionFoster parents are overall more stressed than biological parents, with stress levels increasing over time. Foster children have more behavior problems, and these problems are closely related to their carers' child‐related stress. Further implications for the reduction of parenting stress are discussed.