Families of 99 children with early-onset conduct problems, aged 4–8 years, were randomly assigned to a child training treatment group (CT) utilizing the Incredible Years Dinosaur Social Skills and Problem Solving Curriculum or a waiting-list control group (CON). Post-treatment CT children had significantly fewer externalizing problems at home, less aggression at school, more prosocial behavior with peers, and more positive conflict management strategies than CON children. Significantly more CT than CON children showed clinically significantly improvements on reports and independent observations of aggressive and noncompliant behavior. The differential treatment response was evaluated according to child comorbidity with attention deficit hyperactivity disorder (ADHD), parenting discipline practices, and family risk factors. The only risk factor related to failure to make improvements in child conduct problems after treatment was negative parenting (i.e., maternal critical statements and physical force). The long-term follow-up 1 year later indicated that most of the significant post-treatment changes were maintained.