Cross sectional and M mode echocardiography detected a large right atrial mass in a 70 year old man five days after admission. Three weeks later he developed acute respiratory failure due to a pulmonary embolism. Necropsy showed that the atrial mass was a thrombus. Thus echocardiography should be performed in patients with pulmonary emboli to examine the right cardiac cavities. The need for emergency surgery could then be assessed.