BACKGROUND The papillary muscle (PM) has been reported to be arrhythmogenic focus or the part of the reentrant circuit of ventricular tachycardia (VT) or premature ventricular complexes (PVCs). The purpose of this study is to assess surface 12-lead ECG and electrophysiological characteristics of VT or PVCs originated from PM in either left ventricle (LV) or right ventricle (RV) and to compare them with other idiopathic LV fascicular VT (FVT). METHODS Seven of 195 consecutive patients (3.5%) who underwent ablation for VT or symptomatic PVCs were found to have an ablation site at the PM. Their mean age was 46짹15 years, and mean LV ejection fraction was 51.3 짹11.4 %. The presenting arrhythmia was sustained VT in 4 patients and frequent PVCs in the other 3 patients. The successful ablation site was confirmed by 2-D echocardiography (transesophageal or transthoracic echocardiography) or ventriculography. For comparision, 10 patients with idiopathic FVT (5.1%) were studied. RESULTS Two of 7 patients in the PM group (28.6%) and 2 patients in the FVT (20%) had an evidence of tachycardia-mediated cardiomyopathy. Verapamil was not effective in all VTs in PM group. The PM arrhythmias had a broader QRS complex compared with FVT (149짹7ms vs. 126 짹10ms; P<0.001). Notch of the R wave in the precordial leads was present in all patients in PM group and 3 of 10 patients in the FVT group (P=0.003). VT was not inducible by programmed electrical stimulation in all but 1 in PM group. Purkinje potentials were identified at all ablation sites for FVT, but they were found only in 4 patients with PM group (P=0.02). The ventricular muscle potential preceded the Purkinje potentials in other 3 patients. But there was no significant difference in the local activation time to the onset of the QRS between two groups (35짹3ms vs. 41짹10ms; P=0.14). An open irrigated ablation catheter was used in all but 1 and three-dimensional electroanatomical mapping system was used for 4 patients of PM group. The procedure time was longer than that of FVT(187짹85.5 min vs. 128.7짹65.3 min; P=0.003). Over a mean follow-up period, there was no significant difference in the recurrence rate of ventricular arrhythmia between two groups (14.3% vs. 20.2% ; P=0.76). CONCLUSION Ventricular arrhythmias originating from PM are characterized by broader QRS width with notch of the R wave in the precordial leads, non-responsive to verapamil, with or without preceding Purkinje potential, and the reentrant mechanism less likely. Further study is required to define the mechanism, relationship with Purkinje network, and efficient ablation techniques in this unique subset of VT/PVCs.