This article reports on the initial effects of the Early Impact (EI) Program, a preventive program that includes home and school components designed to arrest the development of conduct problems in preschool children. Participants included 455 preschool-aged children enrolled across 10 schools. Schools were randomly assigned to either EI or control conditions. All participants were screened at the commencement of the academic year to determine those children considered more at a risk for ongoing conduct problems. Following screening n = 66 children were identified in the intervention group and n = 69 in the control. The intervention commenced in the second term of the academic year and ran over a 10-week period. Teachers involved in the intervention were highly engaged and satisfied with the program design. Improvements were reported at the school level at post-intervention with mixed evidence of durability of change at 6-month follow-up. Parent participants were more difficult to engage and reported no changes in the behaviour of children in the home. The study provides initial evidence of the efficacy of the EI intervention as a means of preventing conduct problems in young children at the school level; however, issues associated with parental engagement and home-based change remain to be solved.