This pilot study evaluated the preliminary short- and middle-term impact of a nation-wide, school-based prevention initiative on antisocial behaviour of preschool and primary school pupils in Luxembourg. Seventeen preschool and reception classes (n = 183) were assigned to intervention and comparison conditions. The intervention included social-emotional skills training for both preschool and reception class pupils together with teacher workshops. Three waves of data (pre-test, post-test and 12-month-follow up) were analysed. Univariate analyses of variances with repeated measurements, together with analyses of covariance, for this 2 x 3 design show significant intervention effects: a reduction in oppositional defiant and aggressive behaviour as well as an increase in social and emotional competencies for reception class pupils after a 12-month follow-up period. Preschool pupils displayed an improvement in their social and emotional competencies at post-test, but these effects were lost one year after the intervention was completed. (Contains 7 tables.)