Left bundle branch area pacing (LBBAP) may be associated with greater improvement in left ventricular ejection fraction and reduction in death or heart failure hospitalization when compared with biventricular pacing (BVP) in patients requiring cardiac resynchronization therapy (CRT). We sought to compare the occurrence of sustained ventricular tachycardia or ventricular fibrillation (VT/VF) and new onset atrial fibrillation (AF) in patients undergoing BVP and LBBAP. This International Collaborative LBBAP Study (I-CLAS) included patients with LVEF≤35% who underwent BVP or LBBAP for CRT between Jan 2018 to June 2022 at 15 centers. We performed propensity score matched (PS) analysis of LBBAP and BVP in a 1:1 ratio. We assessed the incidence of VT/VF and new-onset AF among patients with no prior history of AF. Time to sustained VT/VF and time to new-onset AF was analyzed using Cox proportional hazards survival model. Among 1778 patients undergoing CRT (981-BVP, 797-LBBAP), PS matched 1414 patients (PS-BVP:707, PS-LBBAP:707). The occurrence of VT/VF was significantly lower with LBBAP compared with BVP (4.2% vs 9.3%;HR 0.46;95%CI 0.29-0.74;p<0.001). The incidence of VT storm (>3 episodes in 24 hours) was also significantly lower with LBBAP compared with BVP (0.8% vs 2.5%;p=0.013). Among 299 patients with CRT-pacemakers (BVP-111, LBBAP-188), VT/VF occurred in 8 patients in the BVP group vs. none in the LBBAP group (7.2% vs 0%;p<0.001). In 1194 patients with no prior history of VT/VF or antiarrhythmic therapy (BVP-591, LBBAP-603), the occurrence of VT/VF was significantly lower with LBBAP compared with BVP (3.2% vs 7.3%;HR 0.46;95%CI 0.26-0.81;p=0.007). Among patients with no prior history of AF (n=890), the occurrence of new-onset AF >30 seconds was significantly lower with LBBAP compared with BVP (2.8% vs 6.6%;HR 0.34;95%CI 0.16-0.73;p=0.008). The incidence of AF lasting >24 hours was also significantly lower with LBBAP compared with BVP (0.7% vs 2.9%;p=0.015). LBBAP was associated with lower incidence of sustained VT/VF and new-onset AF compared with BVP. This difference remained significant after adjustment for differences in baseline characteristics between patients with BVP and LBBAP. Physiologic resynchronization by LBBAP may be associated with lower risk of arrhythmias compared with BVP.