AbstractThe study evaluated the implementation fidelity and effectiveness of KiVa, an evidence-based program that aims to prevent and address bullying in schools, with a particular emphasis on changing the role of bystanders. The study was a two-arm waitlist control cluster randomized controlled trial in which 22 primary schools (clusters) (N = 3214 students aged 7–11) were allocated using a 1:1 ratio to intervention (KiVa; 11 clusters, n = 1588 students) and a waitlist control (usual school provision; 11 clusters, n = 1892 children)). The trial statistician (but not schools or researchers) remained blind to allocation status. The outcomes were as follows: student-reported victimization (primary outcome) and bullying perpetration; teacher-reported child behavior and emotional well-being; and school absenteeism (administrative records). Implementation fidelity was measured using teacher-completed online records (for class lessons) and independent researcher observations (for school-wide elements). Outcome analyses involved 11 intervention schools (n = 1578 children) and 10 control schools (n = 1636 children). There was no statistically significant effect on the primary outcome of child-reported victimization (adjusted intervention/control OR 0.76; 95% CI 0.55 to 1.06; p = 0.11) or on the secondary outcomes. The impact on victimization was not moderated by child gender, age, or victimization status at baseline. Lesson adherence was good but exposure (lesson length) was lower than the recommended amount, and there was considerable variability in the implementation of whole school elements. The trial found insufficient evidence to conclude that KiVa had an effect on the primary outcome. A larger trial of KiVa in the UK is warranted, however, with attention to issues regarding implementation fidelity. Trial registration: Current Controlled Trials ISRCTN23999021 Date 10-6-13