A 45-year-old man who was diagnosed with idiopathic ventricular fibrillation (IVF) 12 years ago experienced multiple implantable cardioverter defibrillator shocks due to ventricular tachycardia (VT). The recorded electrocardiogram showed VT originating from the right ventricular outflow tract (RVOT). He underwent catheter ablation for VT, and 3-dimensional (3D) mapping revealed a low voltage area in the RVOT. VT was successfully ablated at this site, and his final diagnosis was scar-related VT, not IVF. We suggest that 3D mapping is useful for the reassessment of IVF because an arrhythmic substrate might have been missed in patients who were initially diagnosed with IVF.