Background: The risks of cardiac tamponade and mortality during the first 30 days following catheter or surgical atrial fibrillation (AF) ablation are not well known. Previous large population studies have been limited to in-hospital complication rates and might significantly underestimate the risks of these procedures. Methods: This population based retrospective cohort study was conducted using a large national representative administrative claims database, the Medical Outcomes Research for Effectiveness and Economics Registry (MORE2 Registry®). Thirty-day incidence rates of cardiac tamponade and mortality were calculated and compared by type of procedure (i.e. catheter or surgical), patient gender and age. We analyzed 38,974 AF ablation procedures (catheter: 30,758, 78.9%; surgical: 8,216, 21.1%; age 80 or older: 6,077, 15.6%; 65 to 79: 19,572, 50.2%; 50 to 64: 10,243, 26.3%; 18 to 49: 3,082, 7.9%) performed on 35,754 patients (men: 21,879, 61.2%; women: 13,875, 38.8%) from 2007 to 2012. Results: Thirty-day incidence of cardiac tamponade was 1.74% and differed significantly by type of procedure (catheter: 1.51% vs. surgical: 2.62%, p<0.001) and gender (men: 1.60% vs. women: 1.96%, p=0.010), but not by age group (18-49: 1.49%; 50-64: 1.77%; 65-79: 1.91%; 80 or older: 1.28%, p=0.425). The thirty-day mortality rate was 1.15% and differed significantly by type of procedure (catheter: 0.70% vs. surgical: 2.76%, p<0.001), but not by gender (men: 1.06% vs. women: 1.29%, p=0.058). The mortality rate did not differ significantly from the youngest age group to the next oldest (18 to 49: 0.33% vs. 50 to 64: 0.21%, p=0.315), but increased significantly for the oldest age groups (65 to 79: 1.28%; 80 or older: 2.75%, p<0.001). Conclusion: The 30-day risks of both cardiac tamponade and mortality following catheter AF ablation are greater than the in-hospital rates reported in a previous study. Surgical AF ablation has significantly higher 30-day rates of both cardiac tamponade and mortality, compared to catheter AF ablation. The risk of cardiac tamponade is greater for women than men, but does not vary significantly by age group. Conversely, the risk of mortality is greater for patients age 65 and older, but is not significantly different for women compared to men. These findings provide new comparative safety information that can help inform optimal treatment practices and could serve as the basis of a physician or facility quality measure designed to evaluate relative performance and provide benchmarks to support both consumer choice and quality improvement efforts.