This study examines the effects of the Fast Track program, which is a multicomponent, intensive intervention for children with early-onset conduct problems and continues from 1st grade through high school. Prior research has shown that Fast Track produces small positive effect sizes on children's social and behavioral outcomes at the end of 1st and 3rd grades in comparison to control children. This study addresses the important question of whether this intervention reduces cases of serious problems that can occur during the 4th- and 5th-grade years. Fast Track did have a significant but modest influence on children's rates of social competence and social cognition problems, problems with involvement with deviant peers, and conduct problems in the home and community, compared to children in the control condition. There was no evidence of intervention impact on children's serious problems in the school setting at Grades 4 and 5. This evaluation indicates that Fast Track has continued to influence certain key areas of children's adjustment throughout the elementary school years, reducing children's likelihood of emerging as cases with problems in their social, peer, or home functioning. The stage is set to examine potential prevention effects on these youths' serious antisocial behaviors during adolescence.