The Cardiac failure, Hypertension, Age, Diabetes, Stroke [Doubled](CHADS 2) 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. 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 CHADS 2 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, CHADS 2 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). When using a rhythm control strategy, the CHADS 2 score is a useful scheme for predicting the outcome of AAD treatment of patients with paroxysmal AF.(Circ J 2013; 77: 639–645)