A healthy 55-year-old man was referred for investigation of palpitations. During an episode of palpitation, the ECG documented two types of tachycardia with differing morphologies. One was a narrow QRS complex tachycardia with a heart rate of 140 beats/min. The other was wide QRS complex tachycardia with a heart rate of 210 beats/min. Transformation from one tachycardia to the other occurred spontaneously. Electrophysiological studies revealed two inducible tachycardia, which were shown to represent atrioventricular nodal reentrant tachycardia (AVNRT) and idiopathic left ventricular tachycardia. Radiofrequency catheter ablation of the slow atrioventricular nodal pathway resulted in alleviation of AVNRT. Following the ablation of AVNRT, the wide QRS complex tachycardia was induced during ventricular pacing. The mapping showed that the origin of the ventricular tachycardia was the mid-inferior wall of the left ventricle. Radiofrequency catheter ablation of the ventricular tachycardia resulted in mitigation cure of the idioventricular left ventricular tachycardia.