BACKGROUND AND OBJECTIVES: Chronic atrial fibrillation (AF) causes atrial enlargement and impairs cardiac function. It is known that restoration of sinus rhythm reduces left atrial size and improves left ventricular ejection fraction (EF). However, it's unclear whether the restoration of sinus rhythm is more beneficial than controlling the ventricular rate. This study was designed to compare the effects of two treatment strategies on structural and functional changes of the heart in patients with AF. SUBJECTS AND METHODS: Thirty-seven AF patients who initially cardioverted to sinus rhythm were studied. At 6 months after cardioversion, 17 patients (Group I) maintained normal sinus rhythm (NSR) and 20 (Group II) experienced AF recurrence. Initial and follow-up echocardiography were evaluated in each patient and compared between the two groups. RESULTS: Baseline clinical and echocardiographic findings were similar between the two groups except that the left atrial length was longer in Group II than in Group I. Compared to baseline, left atrial dimension (LAD) and left ventricular mass index (LVMI) measured by M-mode were significantly reduced in group I at follow-up (p<0.05); however, these changes were not observed in group II. The reduction of LAD and LVMI was observed only in patients with less than 50% EF and/or dilated left atrium at baseline (p<0.05). CONCLUSION: In patients with AF, maintenance of NSR after cardioversion significantly reduces LAD and LVMI, especially in cases of left ventricular dysfunction or dilated left atrium. Even after adjustment of baseline LAD, LVMI and EF, maintenance of NSR remains the independent factor that reduces LAD at follow-up (p=0.001).