Conventional posteroanterior chest radiographs of 73 patients with mitral valve disease who had surgery wereanalyzed, and particular attention was directed to the absence of the convexity of the left lower mid cardiacborder (left atrial segment) and to the relation with duration of illness, atrial fibrillation, pulmonarycapillary wedge pressure (PCWP) as well as surgical findings. The results obtained are as follows; 1. The flatnessor concavity of this segment, despite other evidence of left atrial enlargement, was observed in eleven (64.7%) of17 patients who had left atrial thrombosis and six(10.7%) of 56 patients who did not have thrombosis(falsepositive diagnosis). Six (35.3%) of the 17 patients who had left atrial thrombosis did not show this finding onradiographs(false positive diagnosis). Six(35.3%) of the 17 patients who had left atrial thrombosis did not showthis finding on radiographs (false negetive diagnosis). Therefore, the accuracy in the diagnosis of thrombosis ofleft atrium was 64.7%(eleven of seventeen) for patients who had thrombosis of left atrium. 2. The atrialfibrillation was observed in sixteen (94.1%) of 17 patients who had left atrial thrombosis. So, correlationbetween the presence of left atrial thrombosis and atrial fibrillation was evident. But there was no correlationbetween the duration of illness or the PCWP of patients and the presence of left atrial thrombosis.