Cardiac metastases are uncommon and difficult to diagnose clinically; thus, they are most often found only at autopsy. Here we present a case of isolated right atrial cardiac metastasis found 7 weeks after the resection of the primary tumor, which was an adenocarcinoma of the lung. The patient presented with intractable obstructive shock, caused by a ball-valve effect of the atrial lesion that prevented forward blood flow from the right atrium. Computed tomography (CT) scans and echocardiograms failed to detect the lesion, and the patient died 2 weeks later. An autopsy revealed a large, isolated right atrial metastatic adenocarcinoma.