Backgrounds: About 50% of early arrhythmias after pulmonary vein isolation (PVI) of atrial fibrillation (AF) spontaneously dissipate over time and are not due to procedural failure. Recent study suggested that steroid can prevent the recurrence of AF. This study evaluated the effect of single bolus injection of hydrocortisone in the prevention of recurrence after radiofrequency catheter ablation (RFCA) of AF. Methods: We analyzed 229 patients (182 male, 55 짹 11 years) with paroxysmal AF and persistent AF who received steroid (steroid group) and sex and age-matched control (n=57, 44 male, 55 짹 10 years). Mean follow up duration was 12.6 짹 5.6 months. In the steroid group, intravenous hydrocortisone (100 mg) was given the day of RFCA. Catheter ablation technique included PVI and cavotricuspid isthmus block with or without additional ablation. Results: The patients with immediate AF recurrence (��� 3 days after the RFCA) was 19 (14%) and 12 (13%) in control and steroid groups (p=0.86), respectively. C-reactive protein was lower in steroid group than in control group, however there was no significantly different (11 짹 8 vs 26짹 20 mg/L, p=0.07). There was also no difference in body temperature (37.0 짹 0.3 Vs. 37.1 짹 0.5 oC, p=0.61) and WBC count (8789 짹 2281 vs. 9135 짹 2523/���, p=0.47) initial 3 days after ablation between steroid and control group during the. The rate of infection and hematoma was not different between 2 groups. Single bolus injection of hydrocortisone did not decrease AF recurrences between 4 and 30 days after ablation (13% vs. 14%, p=0.85). The AF-recurrence free rate at 12 months post-ablation was not different between two groups (77% vs. 75%, p=0.8 by the log-rank test). Conclusions: Low dose single bolus injection of hydrocortisone shortly after AF ablation was not effective for preventing immediate and AF recurrences during mid-term follow-up period after AF ablation.