The present study reports the short- and long-term effects of an anti-bullying intervention program based on a particular set of curricular activities that aimed to create classroom opportunities for awareness raising, self-reflection, and problem-solving situations relevant to bullying. The core of the intervention was a four-week period during which a series of activities were organised in each individual class. An experimental pre-test/post-test design with a control group was used. The sample consisted of 454 pupils (206 control: 123 boys and 83 girls; and 248 experimental: 126 boys and 122 girls) drawn from the fourth- to sixth-grade classrooms of 10 primary schools in central Greece (mean age = 10.23, SD = 0.84). Data were collected using self-report measures, before the intervention (December 2003), immediately after the intervention, at the end of the same school year (May 2004), and six months afterwards (November 2004). The results indicated that the program contributed to a positive reduction in outsider behaviour (children remaining uninvolved and thus silently allowing bullying to continue) and enhanced students' self-efficacy beliefs for both assertion and intervening in bully/victim incidents. However, the long-term effectiveness of the program was limited. These findings have important implications for interventions to tackle the negative effects associated with bullying in schools. (Contains 3 tables.)