Background: There is a close anatomical relationship between aortic sinus cusp (ASC) and the left pulmonary sinuscusp (LPSC). Purpose: The study was to investigate the significance of mapping and ablation of LPSC for ventricular arrhythmias (VAs) with QRS morphology resembling that of ASC arrhythmias. Methods: In 33 consecutive patients undergoing successful catheter ablation of idiopathic VAs with ASC arrhythmia-like QRS morphology, LPSC was mapped carefully. Results: Among the 33 patients studied, QRS morphology resembled that of arrhythmias of either right or left coronary cusp (LCC or RCC) origin in 17 and 16 patients, respectively. Out of 12 arrhythmias with the earliest potential in LCC during left-sided mapping, an earlier potential in LPSC than in LCC was recorded in 2 arrhythmias, the same earliest potential in LPSC as in LCC was recorded in 6 arrhythmias, and 5 (42%) were finally ablated successfully in LPSC using the reversed U curve. Out of 16 arrhythmias with the earliest potential in the RCC during left-sided mapping, an earlier potential in LPSC than in RCC was recorded in 4 arrhythmias, the same earliest potential in LPSC as in RCC was recorded in 3 arrhythmias, and 4 (25%) were finally ablated successfully in LPSC using the reversed U curve. Conclusions: For VAs with ASC arrhythmia-like QRS morphology, mapping in LPSC could have unique electrophysiologic characteristics, and some of them could be eliminated in LPSC using reversed U curve.