To determine the effectiveness of school-based interventions that focus on changing dietary intake and physical activity levels to prevent childhood obesity. MEDLINE and EMBASE were searched (January 2006 to September 2007) for controlled trials of school-based lifestyle interventions, minimum duration of 12 weeks, reporting weight outcome. Thirty-eight studies were included; 15 new studies and 23 studies included within the National Institute for Health and Clinical Excellence obesity guidance. One of three diet studies, five of 15 physical activity studies and nine of 20 combined diet and physical activity studies demonstrated significant and positive differences between intervention and control for body mass index. There is insufficient evidence to assess the effectiveness of dietary interventions or diet vs. physical activity interventions. School-based physical activity interventions may help children maintain a healthy weight but the results are inconsistent and short-term. Physical activity interventions may be more successful in younger children and in girls. Studies were heterogeneous, making it difficult to generalize about what interventions are effective. The findings are inconsistent, but overall suggest that combined diet and physical activity school-based interventions may help prevent children becoming overweight in the long term. Physical activity interventions, particularly in girls in primary schools, may help to prevent these children from becoming overweight in the short term.