INTRODUCTION: Deep inspiratory breath-hold (DIBH) has become standard in radiotherapy for left-sided breast cancer to reduce the heart dose. This study evaluated breath-hold stability and reproducibility using Elekta's Active Breathing Coordinator™ (ABC) and its effectiveness and feasibility in left-sided breast cancer patients undergoing radiotherapy. METHODS: Eligible patients were planned with free breathing (FB) and DIBH protocols. DIBH treatment was considered if the mean heart dose (MHD) was ≥2 Gy on the FB plan. Those who proceeded with DIBH treatment were enrolled for the pilot study. Electronic portal images of DIBH treatment beams were taken using the movie-exposure mode for breath-hold stability and reproducibility analysis. DIBH effectiveness in heart dose reduction and impact on simulation and treatment durations were compared with FB protocol. RESULTS: Out of 56 eligible patients, 15 proceeded with DIBH treatment. The mean difference of patient setup within a single breath-hold was 0.4 mm; between different breath-holds of the same beam 1.1 mm and between different days 2.6 mm. DIBH reduced the MHD by 47% and the mean left anterior descending artery (LAD) dose by 35%. DIBH took longer time than FB in simulation and treatment. At least 14% of the eligible patients did not tolerate DIBH during simulation. CONCLUSIONS: ABC leads to stable and reproducible breath-holds and results in significant heart dose reductions. It may not be tolerated by all patients and has resource implications.