Conventional posteroanterior chest radiographs of 73 patients with mitral valve disease who had surgery were analyzed, and particular attention was directed to the absence of the convexity of the left lower mid cardiac border (left atrial segment) and to the relation with duration of illness, atrial fibrillation, pulmonary capillary wedge pressure (PCWP) as well as surgical findings. The results obtained are as follows: 1. The flatness or concavity of this segment, despite other evidence of left atrial enlargement, was observed in eleven (64.7%) of 17 patients who had left atrial thromosis and in six (10.7%) of 56 patients who did not have thrombosis (false positive diagnosis) Six(35.3%) of the 17 patients who had left atrial thrombosis did not show this finding on radiographs (false negative diagnosis). Therefore, the accuracy in the diagnosis of thrombosis of left atrium was 64.7% (eleven of seventeen) for patients who had thrombosis of left atrium. 2. The atrial fibrillation was observed in sixteen (94.1%) of 17 patients who had left atrial thrombosis. So, correlation between the presence of left atrial thrombosis and atrial fibrillation was evident. But there was no correlation between the duration of illness or the PCWP of patients and the presence of left atrial thrombosis.