We evaluated the efficacy of a manualized multimodal treatment program for young externalizing children. Families were assigned randomly to an immediate 12-week parent and child treatment condition (n = 24) or to a delayed-treatment condition (n = 23). Parents had high attendance, high satisfaction with treatment, and increased knowledge of behavior management principles. Relative to the waitlist condition, treatment parents reported statistically and clinically significant reductions in child behavior problems, improved parenting practices (i.e., increased consistency, decreased power assertive techniques), an increased sense of efficacy, and reduced parenting stress. There was a trend toward parents improving their attitudes toward their children. In considering the process of change, we found evidence that improved parenting practices mediated reductions in child behavior problems and that child improvements mediated changes in parent attitudes and stress. Five months following treatment, teachers reported significant improvements in child behaviors, whereas parents reported that reductions in child behavior problems and parenting stress were maintained.