In an attempt to define quantitatively the relation between left atrial size and atrial fibrillation, echocardiography was used to study 58 patients with mitral stenosis and sinus rhythm, 22 patients with mitral stenosis and newly appeared atrial fibrillation, 62 patients with mitral stenosis and atrial fibrillation, 14 patients with sinus rhythm who undergone mitral valve replacement, 18 patients with atrial fibrillation who undergone mitral valve replacement,17 patients with idiopathic atrial fibrillation and 22 control persons. In all groups of mitral stenosis, atrial fibrillation was rare when left atrial dimension was below 40mm but common when this dimension exceeded 40 mm. When left atrial dimension exceeded 50 mm, sinus rhythm was rare even in patients who undergone mitral replacement operation. These data suggest that left atrial size is an important factor in the development of atrial fibrillation. To reduce the risk of systemic embolism complicating atrial fibrillation and to reduce the need of long term anticoagulant therapy postoperatively, the left atrial dimension should be followed closely for the patients with mitral stenosis.