The authors report a fifty-year-old woman with acute inferior myocardial infarction in whom left atrial mobile thrombus was found by echocardiography. They believe this to be the first echocardiographic documentation of such a thrombus after acute myocardial infarction. The combination of the stagnant flow in the left atrium due to atrial ischemia, the low output state due to left ventricular dysfunction, and the hemoconcentric tendency from use of diuretics might have played a role in the present thrombus formation. The thrombus was safely lysed without complication after intravenous urokinase.