Implantable cardioverter-defibrillators (ICDs) improve the survival of patients with ischemic or non-ischemic cardiomyopathy and a reduced ejection fraction. However, the efficacy of ICD therapy in patients with right ventricular outflow tract ventricular tachycardia (RVOT-VT) and early stage arrhythmogenic right ventricular dysplasia / cardiomyopathy (ARVD/C) has not been well clarified. Although the prognosis of RVOT-VT is generally good, malignant forms of RVOT-VT resulting in polymorphic VT have been reported by several investigators. Radiofrequency catheter ablation is still effective in such patients, and thus an ICD implantation is usually not required. On the other hand, according to the current guideline in patients with ARVD/C, an ICD implantation is recommended for secondary prevention when the patients develop sustained VT of VF. An ICD implantation may also be considered for primary prevention in high-risk patients: extensive disease, family history of sudden cardiac death, or undiagnosed syncope. Since an ICD implantation in the early stage of ARVD/C is controversial, physicians should well consider its risks and benefit. Early intervention with ICD therapy in ARVD/C patients may reduce the arrhythmic death but increase the device related complications especially in younger patients.