BACKGROUND: This study investigated the prevalence, electrocardiographic and electrophysiological characteristics, and ablation outcome of idiopathic ventricular arrhythmias (VAs) originating from the infundibular muscles (IFMs) in the right ventricle consisting of the parietal band (PB) and septal band (SB). METHODS AND RESULTS: We studied 19 patients with idiopathic VA origins in the PB in 14 and SB in 5 among 294 consecutive patients with VA origins in the right ventricle. PB and SB VAs exhibited left bundle branch block with a left inferior (n=12) or superior (n=2) axis and left (n=4) or right inferior (n=1) axis pattern, respectively. In lead I, all PB VAs exhibited R waves while SB VAs often exhibited S waves. A QS pattern in lead aVR and the presence of a notch in the mid-QRS were common in all IFMs VAs. During IFMs VAs, a far-field ventricular electrogram with an early activation was always recorded in the His bundle region regardless of the location of the VA origins. With 9.2±6.9 radiofrequency applications and a duration of 972±946 seconds, catheter ablation was successful in 15 patients. VAs recurred in 4 during a follow-up period of 43±24 months. A change in the QRS morphology was observed spontaneously in 5 patients, immediately after ablation in 4, and at the time of the VA recurrence in 2. CONCLUSIONS: Idiopathic VAs originating from the IFMs are rare (PB>SB). Catheter ablation of these IFMs VAs was challenging, requiring a large amount of the radiofrequency energy delivery for a successful ablation with a relatively high recurrence rate.