Children in foster and residential care have a higher risk of developing a reactive attachment disorder and mental health problems in general than do children in the general population. However, the relationship between mental health problems and attachment disorder behaviors in looked-after children has not been given the necessary attention in German-speaking countries to date. We conducted an epidemiological questionnaire study to compare looked-after children with children from the general population. The total sample consisted of 731 children. This included 116 children in residential care (M = 13.5 years, SD = 2.73; 66.4% male), 276 foster children (M = 10.2 years, SD = 3:40; 56.2% male), and 339 children from the general population (M = 7.4 years, SD = 2.31; 49% male). Children’s mental health problems were assessed with the German version of the Child Behavior Checklist (CBCL), while reactive attachment disorder behaviors were screened with the German translation of the Relationship Problems Questionnaire (RPQ). These questionnaires were answered by the caregivers who spend the most time with the children. For children from the general population and foster care, paper-based questionnaires were completed by the biological parents and foster parents, respectively. The primary caregivers of children in residential care completed computer-based questionnaires. The CBCL scores of the three groups differed significantly, F(2, 724) = 185.45, p < .001. Looked-after children scored significantly higher than children from the general population, t(615.44) = 20.75; p < .001, and children in residential care had higher scores than foster children, t(291.40) = 2.97, p < .01. In all, 82.1% of children in residential care, 63.8% of foster children, and 18.0% of the control group scored 60 or higher on the CBCL. Children in residential care and children in foster care were 20.96 (95% CI: 12.01-36.60) and 8.02 (95% CI: 5.54-11.61) times more likely to have a high score in the CBCL than children in the general population. The RPQ scores of the three groups also differed significantly, F(2,738) = 123.59, p < .001. Looked-after children had significantly higher RPQ scores than children from the general population, t(352.98) = 4.17; p = .001. However, there were no differences between children in residential care and children in foster care, t(271.29) = 1.06; p = .29. We found that 37.9% of the children in residential care, 31.9% of the children in foster care, and 2.4% of the control group scored 7 or higher on the RPQ. Children in residential care were 25.29 (95% CI: 11.42-56.01) times and children in foster care were 19.37 (95% CI: 9.19-40.82) times more likely to score high on the RPQ than children in the general population. There were significant correlations for all scales of the CBCL and the RPQ. Looked-after children had more mental health problems and more attachment disorder behaviors than children from the general population. An early assessment of looked-after children's mental health problems and attachment disorder behaviors is essential to carefully tailor a permanency plan to a child's needs.