The case of a 55-year-old man with LV dysfunction in whom the baseline cardiac rhythm falsely mimicked a sinus rhythm (SR) but actually originated from the left superior and inferior pulmonary vein (PV) is reported. The P waves before ablation were flat in leads I and V1, negative in lead aVL, and positive in leads II, III, aVF. After the left superior PV was isolated from the left atrium, another ectopic rhythm newly appeared from the left inferior PV. Interestingly, the LV systolic function improved after the resumption of the SR, thus suggesting that tachycardia-induced cardiomyopathy might be involved in the mechanism of LV systolic disturbance.