Converging evidence indicates that maltreated foster children have more behavior problems and poorer peer relations than nonmaltreated, biologically reared youths. However, it is unclear whether deficits in peer relations operate independently or as a result of increased behavior problems and whether outcomes for foster children differ by sex. To address these questions, multi-informant methods were used to assess peer relations at school entry among maltreated foster children and a comparison sample of low-income, nonmaltreated, biologically reared children (N = 121). Controlling for caregiver-reported behavior problems prior to school entry, results from a multigroup structural equation modeling analysis suggested significant relationships between foster care status and poor peer relations at school entry and between foster care status and level of behavior problems prior to school entry for girls only. These Sex * Foster Care Status interactions indicate that the unique needs of each sex should be addressed when intervening with maltreated foster children.