The positive of the Norwegian anti-bullying programme (Olweus, 1992) stimulated other countries to tackle bully/victim problems. However, outcome studies found rather low levels of effect or even inconsistent results. The purpose of the present study was to evaluate behavioural effectiveness of a school-based anti-bullying approach within Flemish schools. In addition, specific attention was given to the relationship between outcome findings and external support. A total of 18 schools, comprising 1104 primary and secondary school children were recruited for this study. Students ranged in age from 10 to 16 years. For this study, an experimental pre-test/post-test design was used which included a control group. Three groups were established. The first group, Treatment with Support, involved students from schools that implemented a school-based anti-bullying intervention with additional support from the research group. The second group, Treatment without Support, also involved students from schools that implemented a school-based anti-bullying programme. However, in contrast with those falling under the first condition, this group of schools could not appeal to the research group for additional help. The last group involved students from schools that did not implement the anti-bullying programme and served as a Control condition. Repeated measures analyses of variance were carried out. The findings regarding the effects of the school-based anti-bullying intervention programme on the extent of bullying and victimisation showed a mixed pattern of positive changes in primary schools and zero outcomes in secondary schools. The findings regarding the effects of external support revealed limited outcomes. The outcomes of the evaluation study confirm that a school-based anti-bullying intervention strategy can be effective in reducing problems with bullying, especially within primary schools. It was argued that the developmental characteristics of secondary school students interfered with the programme outcomes. In addition, the findings revealed no extra effects of specific training sessions.