The article describes the main outcomes of the intervention programme "Positive behaviour, interactions and learning environment in school" (PALS). PALS is an adapted Norwegian version of the School-Wide Positive Behaviour Support model, developed in the United States with school-wide interventions aiming at the prevention of behaviour problems and the promotion of social competence through a supportive learning environment. The effectiveness was evaluated with a quasi-experimental design in four elementary schools, two years after implementation. An equal number of comparison schools were included in the study. Informants were students (n = 735) in third to seventh grade and teachers working at least 50% (n = 82). The immediate reductions in teacher-observed problem behaviour ranged from moderate to large, while the results based on student ratings of social competence and classroom climate were less encouraging. Differential effects were found and implementation quality was together with teacher collective efficacy significantly related to better outcomes. The programme and results are discussed in relation to prior research, limitations of study design, and future implementation and research. (Contains 2 tables.)