BACKGROUND: Mobile right atrial thrombus is an uncommon finding on two-dimensional (2D) echocardiography. Therapeutic alternatives include systemic heparinization, systemic or local thrombolysis, and surgical removal. We report our clinical experience in six patients over a 3-year period (6000 echocardiograms) at a tertiary care referral center. METHODS: There were four men and two women with a mean age of 63 years (range: 47 to 73 years). Indications for echocardiography consisted of progressive dyspnea and chest pain in five patients and syncope with chest pain in one patient. RESULTS: All were observed to have a mobile thrombus in the right atrium. Ventilation perfusion (V/Q) scanning confirmed V/Q mismatch in all patients. Subsequent echocardiography (minutes to 1 day later) in three patients demonstrated absence of the thrombus suggesting pulmonary embolization. One patient died during transesophageal echocardiography (TEE) and autopsy confirmed a large pulmonary embolization in the main pulmonary artery. Treatment consisted of heparinization in 3 patients, systemic thrombolysis in 1 patient, and surgical removal of the thrombus in 1 patient. At surgery, a long serpiginous thrombus was seen in the right atrium, tethered to a fenestrated eustachian valve. There were 3 deaths: 1 patient treated with heparin; 1 patient treated with thrombolysis; and 1 during TEE. Two of the three patients treated with heparin and one patient undergoing surgical removal survived hospitalization. CONCLUSIONS: Mobile thrombus in the right atrium is an unusual echocardiographic finding. It portends a poor prognosis with death due to pulmonary embolism.