A 57-year-old woman showed frequent premature ventricular complexes (PVCs) originating from the right ventricular outflow tract (RVOT), and some of the PVCs triggered polymorphic ventricular tachycardia (PVT). Structural heart diseases were ruled out by conventional cardiac examinations. Radiofrequency catheter ablation was successful in eliminating the PVCs and subsequent PVT. However, epinephrine infusion unmasked her prolonged QT interval, and a genetic analysis revealed a KCNH2 mutation (R694H) as the cause of latent type-2 long QT syndrome (LQTS). This case suggests that latent LQTS may work as an arrhythmogenic substrate of PVT triggered by a benign form of RVOT-PVCs in patients with a structurally normal heart.