Despite recent research indicating that 1 of the pivotal times for identifying pathways to early conduct problems is the toddler period, few family-based preventive interventions have been specifically designed to modify child disruptive behavior during this age period. This randomized trial tested the effectiveness of the Family Check-Up in sustaining maternal involvement and preventing the exacerbation of child conduct problems among 120 at-risk toddler-age boys, half of whom were randomly assigned to a treatment condition. The intervention was associated with reductions in disruptive behavior and greater maternal involvement and was particularly effective for children at greater risk for a persistent trajectory of conduct problems. The results are discussed in relation to other preventive interventions for young children.