Prethrombotic state was investigated in the patients with mitral stenosis (MS) from the viewpoint of hypercoagulability. Among of factors concerning in hypercoagulability, a decrease in level of antithrombin III (AT III) in plasma, which is the most potent anticoagulant in blood, is regarded to be very important to hypercoagulability. Therefore, to investigate hypercoagulable state in the patients with MS, the concentration of AT III in plasma was measured using single radial immunodiffusion method. The level of AT III in plasma was decreased in the patients with MS, and the severer the degree of MS, the lower the level of AT III in plasma. A decrease in level of AT III in plasma in subjects with sinus rhythm and atrial fibrillation was detected not only in cases of MS but also those of other valvular diseases or atrial fibrillation with non-rheumatic heart diseases (RHD). In congestive subjects, the level of AT III in plasma was decreased specifically in cases of MS contrasted with those of other valvular diseases or non-RHD. It is concluded that the low level of AT III in plasma contributes much to prethrombotic state in the patients with MS, combined with endcardial damage due to RHD and with blood flow disturbance by MS, of and congestion.