Background: A cluster randomized controlled trial was conducted to assess the effectiveness of the Active Classrooms intervention, which integrates movement into academic lessons, on the moderate-to-vigorous physical activity levels (MVPA) of primary school children during class-time and throughout the school day. Methods: Ten classroom teachers and their students aged 8 to 12 years were recruited and randomized into the Active Classrooms intervention group (n = 131students, n = 5teachers) or a delayed-treatment controlled group (n = 117students, n = 5teachers). The intervention group participated in active academic lessons taught by the classroom teacher over an 8 week period. Accelerometers were used to gather physical activity data at baseline, postintervention and at 4 months follow-up. Teachers completed a questionnaire to evaluate the program. Results: A significant difference for change in daily class time MVPA levels was identified between the treatment (n = 95) and control (n = 91) groups from pre- to postintervention (P < .001) and this difference was maintained at follow-up (P < .001). No significant difference emerged between the treatment and control groups for change in school day MVPA levels from pre- to postintervention (P = .52) or follow-up (P = .09). Teachers reported that they were highly satisfied with the program. Conclusions: Movement integration has the potential to improve physical activity levels of primary school children in the classroom.