Little information is available concerning clinical profiles and outcomes of treatment in Japanese patients with first detected atrial fibrillation (AF). In the present study, 459 patients with paroxysmal AF (309 males, mean age, 66 ± 12 years) were divided into a first detected AF group (group A, n = 143) and a non-first detected AF group (group B, n = 316). Clinical profiles, prophylactic efficacy of antiarrhythmic drug therapy (AAD), and cardiovascular prognosis during a mean follow-up period of 50 ± 35 months were compared between the two groups. The number of AF recurrences in the individual patients regardless of AAD were significantly lower in group A than in group B (0.8 ± 1.4 versus 1.7 ± 1.9)(P < 0.05). The percentages of patients free from conversion to chronic AF at 12, 36, 60, and 120 months were significantly higher in group A (98%, 96%, 93%, and 91%, respectively) than in group B (95%, 86%, 83%, and 79%, respectively)(P < 0.01). The annual rates of hospitalization for thromboembolism, heart failure, and cardiovascular death did not differ between group A (2.2%, 1.1% and 1.0%, respectively) and group B (2.2%, 1.9% and 1.1%, respectively). In multivariate logistic regression analysis, a CHADS₂ score ≥ 2 points (odds ratio 13.1, 95% confidence interval 3.36-51.0, P = 0.001), nocturnal AF onset (OR 0.201, 95% CI 0.050-0.815, P = 0.025), left ventricular diastolic dimension (LVDd) ≥ 50 mm (OR 3.845, 95% CI 1.078-13.71, P = 0.038), and conversion to chronic AF (OR 3.547, 95% CI 1.002-13.64, P = 0.048) were associated with cardiovascular events in group A. Rhythm control therapy with antiarrhythmic drugs was shown to be more effective for patients in group A than in group B. It is particularly important to prevent cardiovascular events in first detected AF patients with a CHADS₂ score ≥ 2 points, LVDd ≥ 50 mm, and conversion to chronic AF.