Aims To investigate the technical feasibility of utilizing the Active Breathing Coordinator for planning of postoperative three-dimensional conformal radiation therapy in patients with early stage breast cancer undergoing breast conservation therapy. Methods Patients with early stage breast cancer for whom adjuvant radiotherapy after breast-conserving surgery was planned were consecutively enrolled. Five sessions of simulation with the Active Breathing Coordinator were planned for each patient. Computed tomography for simulation was not acquired until a good level of compliance with the procedure was achieved by the patient. Patients who did not show a satisfactory level of compliance after the planned fifth session were defined as noncom-pliant. Two simulation computed tomography scans were acquired: the first without the Active Breathing Coordinator during free breathing, the second with the Active Breathing Coordinator. Forward intensity-modulated treatment plans were calculated. Mean lung dose (MLD_ipsilateral) and V30 (V30_lung) for the ipsilateral lung and V30 for the heart (V30_heart), were evaluated. Results Twenty consecutive patients were enrolled (6 with left-sided breast cancer and 14 with right-sided breast cancer). Eighteen of the patients completed the simulation computed tomography with the Active Breathing Coordinator after 1–5 sessions (median, 3). In 16 of the 18 patients, a reduction of V30_lung was observed with the Active Breathing Coordinator. In 15 of the 18 patients, a reduction of MLD_ipsilateral was also observed. In 5 of the 6 patients with left-sided breast cancer, a reduction of V30_heart was noted. Conclusions Routine application of the Active Breathing Coordinator in clinical practice is feasible, even though it requires an increased workload. Dosimetric results are encouraging in terms of a better sparing of the ipsilateral lung and the heart.