Objective The objective of this study was to test whether heart-rate feedback prompts increased physical activity (PA) in primary school students. Methods A controlled trial with 210 9–11 year olds from subsidized Hong Kong Government primary schools was completed in December 2006. Schools were randomly assigned to one of three groups: Control (CG); Educational program (EG); No-educational program (NEG). Intervention In a short-term school-based intervention, heart-rate feedback was given either in combination with an educational program (EG) or in isolation (NEG). Long-term retention was assessed in comparison to the control group after 6 months follow-up. Main outcome measures Physical activity and changes in attraction to PA. Results In the short-term, heart-rate monitor feedback increased total daily PA by an average of 24% (p < 0.001) and vigorous PA by 0.6% (p < 0.05). No change was found for moderate PA. Increases occurred regardless of involvement in the educational program, and in the absence of any change in attitudes towards PA. No long-term retention was apparent. Conclusions The findings from the study suggest that feedback from heart-rate monitors encourages low intensity PA, but when removed increases do not persist.