The aims of this study were to examine the efficacy of the Incredible Years program (IY) with Portuguese families of preschoolers, moderator and mediator effects, and sustainability of results. Design: randomized controlled design with pre- and post-intervention, and 12 and 18 months assessments. Participants: 124 children aged 3-6 years, at risk of disruptive behavior problems. Children in the IY group showed significant reduction in behavior problems and increase in social skills; caregivers improved parenting practices and self-confidence. Positive clinical and functional impacts were demonstrated. IY was efficacious with a wide range of families. The moderating effect of the child’s age suggests that IY prevents a decrease in social skills for the ages covered by this study. Changes in parental self-efficacy affected changes in parental practices, promoting changes in children’s behavior. Positive effects were maintained over time.