Low levels of physical activity coupled with high levels of television viewing have been linked with obesity in children. The objective of this study was to assess the efficacy of 'Switch Off-Get Active', a 16-week controlled health education intervention, in increasing physical activity and reducing screen time and BMI in primary school children. A secondary objective was to compare children with high and low screen time. Participants were 312 children aged 10.2+/-0.7 years, attending nine schools in areas of social disadvantage. The 10-lesson, teacher-led intervention, conducted in spring 2003, emphasised self-monitoring, budgeting of time and selective viewing. Differences, adjusted for baseline values by ANCOVA, existed between intervention and control children at follow-up for self-reported physical activity (intervention +0.84 30 min blocks/day, 95%CI 0.11-1.57, p<0.05) and self-efficacy for physical activity (p<0.05) but not self-reported screen time (intervention--0.41 blocks/day, 95%CI--0.93-0.12, p=0.13) or BMI (p=0.63). Cross-sectional comparisons at baseline indicated lower physical activity, self-efficacy for physical activity and aerobic fitness and a higher BMI in children with high screen time. In conclusion, health education interventions can increase physical activity in primary school children but follow-ups of longer duration may be needed to demonstrate intervention effects on BMI.