The presence of left atrial thrombus in mitral stenosis has been reported to be associated with several factors. These are age, sex, presence of atrial fibrillation, episodes of congestave heart failure, calcification of mitral valve, embolic episode, etc. Since none of these single factor has been always related to the presence of left atrial thrombus, related risk factors to left atrial thrombosis were studied in patients with mitral stenosis using chi square test. We had operated on 191 cases of mitral valvular heart disease from Jan. 1978 to June 1981 at Severance Hospital, Yunsei University College of Medicine. The left atrial thrombi were present in 41 cases among 191 cases of mitral valvular heart disease and it was present in 31 cases among 89 cases of pure mitral stenosis. Only 10 cases among 74 cases of mitral stenoregurgitation had left atrlal thrombi, whereas no left atrlal thrombus was found in patients with pure mitral regurgitation. Related risk factors studied herein were sex, episodes of congestive heart failure, atrial fibrillation, pulmonary capillary wedge pressure, mitral valve area calculated by Gorlin and Gorlin`s formula cardiac output and left atrial dimension by echocardiogram. In this study presence of atrial fibrillation was deemed to be one of the most potential risk factors and other factors of age, duration of symptoms, episode of embolization, calclfication of mitral valve, associated aortic and tricuspid valve disease, ejection fraction of left ventricle by cineangiocardiogram and echocardiogram were not significantly related to the presence of left atrlal thrombi in a statistical viewpoint.[KTCS 1982;2:204-212]