Objective: The effects of school-based antibullying programs have typically been examined on small samples, with number of schools ranging from 1 to 78 (Farrington & Ttofi, 2009). This study investigated the effectiveness of the KiVa antibullying program in the beginning of its nationwide implementation in Finland. Method: At each time point, the participants included 888 schools with approximately 150,000 students in 11,200 classrooms in Grades 1–9 (8–16 years of age; 51% boys and 49% girls). Victims and bullies were identified with the global questions from the Revised Olweus Bully/Victim Questionnaire (Olweus, 1996), utilizing the criteria suggested by Solberg and Olweus (2003). The program effects were examined by calculating odds ratios based on a cohort-longitudinal design, correcting the standard errors for clustering. Results: During the first 9 months of implementation, the KiVa program reduced both victimization and bullying, with a control/intervention group odds ratio of 1.22 (95% CI [1.19, 1.24]) for victimization and 1.18 (95% CI [1.15, 1.21]) for bullying. Conclusions: Generalized to the Finnish population of 500,000 students, this would mean a reduction of approximately 7,500 bullies and 12,500 victims.