Bullying is an internationally recognized problem and school-based bullying is particularly pervasive. KiVa is a robustly evidenced school-based antibullying programme developed and evaluated at Turku University, Finland, and subsequently disseminated across Finland. Following a positive UK trial of Unit 2 (for 10- to 12-year-olds), further UK dissemination has taken place. This study presents (a) pupil self-reported levels of victimization and bullying prior to, and after, one year of KiVa implementation (Units 1 and 2) with 7- to 11-year-olds from 41 schools, and (b) programme training and delivery costs. Data from 41 primary schools were analysed using a linear mixed model effects analysis. Results revealed statistically significant reductions in victimization and bullying after one year of programme implementation. Ongoing costs were small, at £2.84 per Key Stage 2 pupil per annum. These promising results highlight the need for further more rigorous evaluation of KiVa in the UK, including the exploration of factors associated with effective implementation, and the importance for educators and policy makers of evaluating both impact and costs when implementing programmes to prevent and reduce bullying.