Background: Electrical cardioversion (ECV) is a common procedure for terminating atrial fibrillation (AF). ECV is associated with brady-arrhythmic events, however, the age-specific risks of clinically significant brady-arrhythmic events are unknown. Methods: Using Danish nationwide registers, we identified patients with AF at their first non-emergent ECV between 2005 and 2018 and estimated their 30-day risk of brady-arrhythmic events. Moreover, factors associated with increased risks of brady-arrhythmias were identified. Absolute risks were estimated using logistic regression models fitted with natural splines as well as standardization (G-formula). Results: We identified 20,725 eligible patients with a median age of 66 years (IQR 60-72) and most males (73%). The 30-day risks of brady-arrhythmic events after ECV were highly dependent on age with estimated risks ranging from 0.5% (95% CI 0.2-1.7) and 1.2% (95% CI 0.99-1.5) to 2.7% (95% CI 2.1-3.3) and 5.1% (95% CI 2.6-9.7) in patients aged 40, 65, 80, and 90 years, respectively. Factors associated with brady-arrhythmias were generally related to cardiovascular disease (eg, ischemic heart disease, heart failure, valvular AF) or a history of syncope. We found no indications that pre-treatment with anti-arrhythmic drugs conferred increased risks of brady-arrhythmic events (standardized absolute risk difference -0.25% [95% CI -0.67 to 0.17]). Conclusions: ECV conferred clinically relevant 30-day risks of brady-arrhythmic events, especially in older patients. Anti-arrhythmic drug treatment was not found to increase the risk of brady-arrhythmias. Given the widespread use of ECV, these data should provide insights regarding the potential risks of brady-arrhythmic events.