The current study was designed to explore the use of behavioral (i.e., accuracy and reaction times) and electrophysiological measures (i.e., event-related potentials) to assess the impact of a family-based preventive intervention for preschool-aged, maltreated children in foster care. These measures were recorded during a computerized flanker task designed to assess cognitive control and response monitoring. The sample was recruited from a larger randomized efficacy trial of Multidimensional Treatment Foster Care for Preschoolers (MTFC-P) and included foster children assigned to the intervention condition (n=10), foster children assigned to a services-as-usual comparison condition (n=13), and low-income, nonmaltreated community children (n=11). The children's behavioral and electrophysiological performance on the task was generally consistent with previous research with adults and older children. There were no group differences on the behavioral measures of cognitive control or response monitoring. Notably, however, group differences were observed on the electrophysiological measures of response monitoring. Specifically, the foster children who received services as usual were significantly less responsive to performance feedback about errors than the foster children who received the intervention and the nonmaltreated children. Applications of this methodology and implications of the results for future prevention research are discussed.