Background: For elementary school-children with aggressive behaviour problems, there is a strong need for effective preventive interventions to interrupt the developmental trajectory towards more serious behaviour problems. Aim: The aim of this RCT-study was to evaluate a school-based individual tailor-made intervention (Stay Cool Kids), designed to reduce aggressive behaviour in selected children by enhancing cognitive behavioural skills. Method: The sample consisted of 48 schools, with 264 fourth-grade children selected by their teachers because of elevated levels of externalizing behaviour (TRF T-score > 60), randomly assigned to the intervention or no-intervention control condition. Results: The intervention was found to be effective in reducing reactive and proactive aggressive behaviour as reported by children, mothers, fathers or teachers, with effect sizes ranging from .11 to .32. Clinically relevant changes in teacher-rated externalizing behaviour were found: the intervention reduced behaviour problems to (sub) clinical or normative levels for significantly more children than the control condition. Some aspects of problems in social cognitive functioning were reduced and children showed more positive self-perception. Ethnic background and gender moderated intervention effects on child and teacher reported aggression and child response generation. Conclusions: The results of this study demonstrate the effectiveness on outcome behaviour and child cognitions of an individual tailor-made intervention across informants under real-world conditions.