Effects of a peer-led intervention campaign against school bullying, carried out by eight peer counsellors, were studied in an upper-level comprehensive school in southern Finland. The study was conducted in the seventh and eighth grades, the total number of participating classes being 12 and the total number of participants 196 (89 girls and 107 boys). The core of the intervention campaign was a one-week period during which a series of events and activities were organized at the school and in each individual class. In all parts of the campaign, the peer counsellors emphasized each individual's responsibility, as well as their potential to affect whether or not bullying occurs in their school or class. The results showed that the campaign was especially effective among girls. This was most clearly shown by decreases in both self- and peer-reported bullying, as well as an increase in 'power attitudes', i.e. attitude items reflecting the students' self-perceived potential and willingness to influence bullying problems in their class. Among boys, on the other hand, there was a slight decrease in self-reported bullying, not confirmed by peer reports and, unfortunately, there was an increase in pro-bullying attitudes, such as 'bullying might be fun sometimes'. Evaluating the campaign, girls also perceived it as more beneficial and more effective than boys did. Most students who reported being bullied before the intervention were satisfied with the campaign and thought it was helpful.