More than 20,000 children in the Netherlands live in foster families. The majority are in long-term foster family placements, which are intended to provide a stable rearing environment until the children reach adulthood. International studies have shown, however, that compared to children in the general population, foster children have more mental health problems and more negative developmental outcomes in their later life. Less is known about Dutch foster children, however. To fill this knowledge gap, the present study focused on the mental health of 239 foster children (aged 4-12) living in long-term placements in the Netherlands. Their behavior was assessed with the Strengths and Difficulties Questionnaire, which was completed by their foster parents. The results revealed a wide range of problem behavior (ranging from none to very serious problem behavior), and showed that a third of the children have total difficulty scores (TDS) in the clinical range. Higher TDS appear to have a positive univariate association with age of the foster child, age upon entering the current foster family, number of prior foster placements, non-kinship placement, and fostering experience of the foster parents. The more risk factors, the higher the TDS. These findings suggest the importance of the early detection of problems and potential risk factors in foster families, and the need to support a substantial number of foster children and foster families.