To test the hypothesis that atrial enlargement can develop as a consequence of atrial fibrillation, left and right atrial dimensions were measured echocardiographically at two different time points in patients with atrial fibrillation. Patients were selected who initially had normal atrial sizes and who had no evidence of significant structural or functional cardiac abnormalities other than atrial fibrillation either by history or two-dimensional and Doppler echocardiography. Fifteen patients were studied (12 men and three women; mean age, 67.3 years). Average time between studies was 20.6 months. Three orthogonal left atrial dimensions and two right atrial dimensions were measured, and all were found to increase significantly between studies. Also, highly significant increases in calculated left atrial volume (from 45.2 to 64.1 cm3, p less than 0.001) and right atrial volume (from 49.2 to 66.2 cm3, p less than 0.001) were observed. The relative extents of left and right atrial volume increase did not differ, and left ventricular size did not change significantly between studies. These results indicate that atrial enlargement can occur as a consequence of atrial fibrillation. The maintenance of sinus rhythm, therefore, may prevent atrial enlargement and its adverse clinical effects.