The present study was designed to assess the relative contribution of atrial fibrillation and left atrial pressure to changes in the size of the left and right atria in patients with mitral stenosis. The study included 155 subjects, 102 of whom underwent prospective echocardiography and Doppler cardiography, and 69 of whom underwent cardiac catheterization. The size of the atria was determined by two-dimensional echocardiography. There were no significant hemodynamic differences between patients with mitral stenosis who were in either sinus rhythm or atrial fibrillation. The left atrium was larger (p less than 0.001) in patients with mitral stenosis and atrial fibrillation (37.6 +/- 10.8 cm2) than in patients in sinus rhythm (27.8 +/- 7.7 cm2) or normal subjects (15 +/- 3.3 cm2). The size of the right atrium was larger (p less than 0.001) in patients with mitral stenosis and atrial fibrillation (21.7 +/- 5.2 cm2) than in patients in sinus rhythm (13.4 +/- 3.9 cm2) or normal subjects (13.8 +/- 3.7 cm2). Multiple regression analysis showed that the severity of mitral stenosis accounted for 38%, age for 7%, and atrial fibrillation for 11% of the change in the size of the left atrium. Atrial fibrillation accounted for 24%, age for 11, and mitral valve area for 3% of the change in the size of the right atrium. The analysis suggests that the onset of left atrial dilatation in mitral stenosis is the result of an early increase in left atrial pressure. Atrial fibrillation, which develops irrespective of the severity of the mitral stenosis, contributes to a further enlargement of the left and right atria.