The authors describe a case of biatrial thrombosis after acute inferior wall myocardial infarction (MI) with sinus rhythm. The presence of atrial thrombosis in patients with acute MI has recently been recognized with the aid of echocardiography. However, to the author's knowledge, a case of biatrial thrombosis after acute MI has not been reported previously. In this case, blood stasis, resulting from both left ventricular and right ventricular systolic dysfunction, was the most likely cause of the biatrial thrombi formation. In summary, this case suggests that biatrial thrombosis may occur in acute MI, even in the presence of sinus rhythm.