This study investigated the immediate impact of an 8-month center- and home-based prevention program for preschoolers at high risk for conduct problems. We report immediate program effects on observed and self-rated parenting practices and observed child behavior with peers. Ninety-nine preschool-age siblings of adjudicated youths and their families were randomly assigned to an enhanced version of the Incredible Years Series (Webster-Stratton, 1989; n = 50) or to a no-intervention control condition (n = 49). In an intent-to-treat design, the intervention yielded significant effects on negative parenting, parental stimulation for learning, and child social competence with peers. Improvements in negative parenting, stimulation for learning, and child social competence support the potential of the intervention to prevent later conduct problems in high-risk children.