A modified version of the Olweus school bullying questionnaire was administered to a sample of 4,092 pupils, mainly aged 10-12, in ten middle schools, six in the north (Braga) and four in the south of Portugal (Lisbon). We present and discuss the results of this survey on the following topics: frequencies of being bullied and bullying others; types of bullying; places where bullying occurs and children's opinions about the playground. These variables were analysed in terms of factors such as school grades, under-achievement, social class, gender and school location (north or south of the country). Logistic regression was used to identify risk factors for bullying behaviour. For being bullied, an increased risk was found for male and low social class students. After multivariable adjustment, factors remaining significantly associated with bullying others were gender, school grade, social class and years of under-achievement. The results are compared to the results of other studies in Norway, UK, Italy and Ireland.