Objective: To evaluate the effects of an antibullying school intervention in elementary schools. Design: Two-year follow-up randomized intervention group-control group. Setting: Forty-seven elementary schools in the Netherlands. Participants: Three thousand eight hundred sixteen children aged 9 to 12 years. Intervention: During the first study year, an antibullying school program was implemented in the schools in the intervention group. Main Outcome Measures: A questionnaire measuring bullying behavior, depression, psychosomatic complaints, delinquent behavior, and satisfaction with school life and peer relationships was filled out by the students at 3 times to obtain the following data: a baseline measurement, a first-effect measurement at the end of the first year, and a second-effect measurement at the end of the second year. Results: The number of bullied children decreased by 25% in the intervention group compared with the control group (relative risk, 0.75; 95% confidence interval, 0.57-0.98). The intervention group also showed a decline in the scale scores of victimization (-1.06 vs 0.28; P<.01) and active bullying behaviors (-0.47 vs 0.12, P<.05). Self-reported peer relationships also improved in the intervention schools (0.48 vs 0.11; P<.05), and there was a trend for a decrease in reported depression in the intervention schools (-0.33 vs -0.10; P<.10). At follow-up, there were no differences between the intervention and control groups for the outcome measures. Schools had also lowered their antibullying activities during the second study year. Conclusions: An antibullying school policy can reduce bullying behavior. To keep bullying at a consistently low level, schools must continue antibullying measures every year. Continued counseling may help schools in their efforts to establish a lasting antibullying policy. ©2006 American Medical Association. All rights reserved.