Patients with severely depressed left ventricular ejection fractions (LVEFs) receive implantable cardioverter-defibrillators (ICDs) for the primary prevention of sudden death. However, in some patients, LVEFs may improve or even normalize over time, and these patients would no longer be qualified for ICD implantation based on the original criteria for which they have initially received an ICD. We report a patient with idiopathic dilated cardiomyopathy whose LVEF recovered to normal values after pharmacological therapy. Meanwhile, the patient had life-threatening ventricular fibrillation, aborted by the ICD. We reflect on the pathological features of left ventricular reverse remodelling and ventricular arrhythmogenesis, where the myocardial substrate appears to play an important role. Also, after LVEF improvement in a patient with a cardiac device, there is still a debate on whether we should perform a battery replacement.