ICD (implantable cardioverter-defibrillator) has been recently accepted as the most effective treatment for patients with medically refractory or life threatening ventricular tachycardia or ventricular fibrillation. Determination of the DFT (defibrillation threshold) is important in the implantation of ICD. DFT is the lowest shock energy that successfully terminates ventricular fibrillation. Preoperative amiodarone use, increased body size, and left ventricular dilatation are associated with high DFT. Recently, we experienced a 66 year-old male patient presenting with syncope due to idiopathic ventricular fibrillation in whom DFT was high and unstable, probably due to preoperative use of amiodarone.