Background: The Cardiac failure, Hypertension, Age, Diabetes, Stroke [Doubled] (CHADS₂) score is a useful scheme for risk stratification of thromboembolism patients, but there is little information about its usefulness for the evaluation of antiarrhythmic drug (AAD) therapy. Methods and Results: This study included 459 paroxysmal atrial fibrillation (AF) patients (309 men, mean age 66±12 years, mean follow-up 50±35 months) and prophylactic efficacy was analyzed on the basis of CHADS₂ score. (1) Survival rates free from AF recurrence at 1, 6, 12 and 24 months were, respectively, 89%, 74%, 63% and 47% in score-0 group (n=152); 92%, 68%, 59% and 48% in score-1 group (n=158); 86%, 64%, 56% and 46% in score-2 group (n=84); 81%, 65%, 51% and 35% in score-3 group (n=43); and 68%, 50%, 36% and 18% in ≥score-4 group (n=22) (P<0.05; score-0, score-1 or score-2 vs. ≥score-4 group). (2) Survival rates free from progression to chronic AF at 12, 36, 60 and 90 months were, respectively, 95%, 93%, 91% and 89% in score-0 group; 97%, 91%, 89% and 88% in score-1 group; 96%, 93%, 88% and 83% in score-2 group; 91%, 74%, 67% and 60% in score-3 group; and 91%, 82%, 68% and 55% in ≥score-4 group (P<0.01; score-0, score-1 or score-2 vs. ≥score-4 group). (3) In multivariate logistic regression analysis adjusted for potentially confounding variables, CHADS₂ score was associated with AF recurrence (odds ratio [OR] 1.45, 95% confidence interval [CI] 1.16–1.81, P<0.001), and progression to chronic AF during AAD therapy (OR 1.64, 95% CI 1.04–2.69, P<0.001). Conclusions: When using a rhythm control strategy, the CHADS₂ score is a useful scheme for predicting the outcome of AAD treatment of patients with paroxysmal AF.  (Circ J 2013; 77: 639–645)