Idiopathic left ventricular aneurysms and diverticula (LVA/Ds) are rare cardiac malformations that can be detected using certain imaging techniques. Although most patients with these malformations are clinically asymptomatic, some patients exhibit cardiac arrhythmias and other clinical manifestations. In electrophysiological studies, it is often difficult to establish the relationship between clinical manifestations of cardiac arrhythmias and those of LVA/Ds due to anatomical complexities. We herein report the case of a 67-year-old man who was successfully diagnosed with ventricular tachycardia originating from an idiopathic LVA that was clearly demonstrated on a three-dimensional electroanatomical mapping system integrated with CT imaging.