There is little information available on factors affecting the long-term prevention of paroxysmal atrial fibrillation (AF) in the Japanese population. A total of 71 patients (49 men, mean age, 68 ± 8 years) with paroxysmal AF refractory to ≥ 2 class I antiarrhythmic drugs received oral amiodarone (50-200 mg/day). All patients were observed for more than 12 months (mean follow-up period, 47 ± 26 months) and were analyzed on the basis of patient profiles. The percentage of patients with AF recurrence despite amiodarone therapy was 54% in all patients. In multivariate logistic regression analysis adjusted for age and sex, the following factors were associated with preventive efficacy for AF recurrence: left ventricular ejection fraction (LVEF) (relative risk [RR] 0.933, 95% confidence interval [CI] 0.877-0.993, P = 0.029), asymptomatic AF (RR 0.068, CI 0.005-0.870, P = 0.039), and AF occurring irrespective of circadian variation (RR 0.115, CI 0.013-0.988, P = 0.049). The percentage of patients with conversion to permanent AF despite amiodarone therapy was 31% in all patients. In multivariate logistic regression analysis adjusted for age and sex, asymptomatic AF (RR 0.085, CI 0.010-0.732, P = 0.025) was the only factor associated with preventive efficacy for conversion to permanent AF. Amiodarone appears to be effective in maintaining sinus rhythm, especially in patients with impaired left ventricular function. In contrast, amiodarone appears to be refractory in those with asymptomatic AF or AF occurring irrespective of circadian variation.