Background: Effective physical activity (PA) interventions are warranted for youth, and schools have been identified as logical locations for such involvement. Experts and professionals in the field promote comprehensive school PA programs, including classroom PA. The purpose of this study was to determine the effect of a low-cost, teacher-directed classroom-based intervention on the school PA of elementary children. Methods: Nine classroom teachers were provided inexpensive curricula and trained to implement and instruct PA breaks (2 x 30 minute training sessions). The teachers were encouraged to lead 1 activity break per day after the training. One hundred and six elementary students wore pedometers up to 12 days over 3 monitoring periods during the school year (baseline, follow-up, post follow-up) to assess the effectiveness and the sustainability of the intervention. The teachers self-reported the frequency of activity breaks instructed. Results: The teachers (n = 5) who complied with the recommended 1 activity break per day had students who accrued approximately 33% more mean school steps/day at follow-up (approximately 1100) and post follow-up (approximately 1350) compared to controls. Teachers (n = 4) in the intervention who did not comply with the 1 activity break per day recommendation had students accrue similar mean school steps/day as controls. Conclusion: Inexpensive, teacher-directed classroom-based PA interventions can be effective in improving children's PA levels if teachers implement 1 activity break per school day. We recommend promoting the notion of 1 activity break per day in the classroom as part of a comprehensive school PA program that includes quality physical education, recess, and before/after school programs. (Contains 3 figures and 2 tables.)