Background: The aim of the present study was to conduct a preliminary empirical evaluation of the KooLKIDS program, a school-based intervention that utilises a combination of whole-of-class and individual sessions for young children with early onset antisocial behaviour. Method: Trained facilitators delivered the 13-session program to a completer sample of 49 primary school-aged children (7-11 years) with early onset antisocial behaviour (42 boys and seven girls suspended from school or at risk for suspension) and their peers. Results: Analyses showed significant postintervention reductions on many outcome variables, including teacher and child self-rated aggression and antisocial traits. Significant improvements were also found on measures of emotional intensity and peer acceptance. Acceptability and feasibility data were promising. Conclusion: KooLKIDS has the potential for redirecting the trajectory of antisocial behaviour towards more positive social and emotional outcomes for these at-risk children.