When breast cancer arises on the side of an implantable cardioverter-defibrillator (ICD), the device may have to be removed for surgery, and this is hazardous in congenital long QT syndrome (LQTS). A 51-year-old woman with congenital LQTS, a prior aborted sudden cardiac arrest, and an appropriate ICD shock was scheduled for left total mastectomy with sentinel lymph node biopsy (final pathology pT2N0, ER-positive, PR-positive, HER2-negative). The left-sided ICD generator was removed two days before surgery, and an external defibrillator was kept on standby. Isoproterenol was infused at 1 mcg/min to keep the heart rate at 60-80 bpm. Surgery was done under propofol-remifentanil total intravenous anesthesia with external defibrillator pads, without QT-prolonging premedication. A new generator was implanted in the same field before emergence. Intermittent premature ventricular contraction (PVC) bigeminy occurred without hemodynamic change. By monitor trend, PVCs averaged 7.0/min with the native rhythm (13:00-15:46), 8.8/min during ventricular pacing at 70 bpm, and almost none after atrial pacing at 80 bpm was set. No sustained ventricular arrhythmia occurred, and she was discharged on postoperative day 9, with no device or arrhythmia events during follow-up to August 19 (11 weeks). Removal of the ICD for mastectomy can be done in LQTS with a prepared protection plan, although arterial pressure and electrolyte monitoring should be added.