Two cases of large, free-floating right heart thrombi in patients with massive pulmonary embolism and a history of deep vein thrombosis are presented. In a 30-year-old male with prominent obesity and a history of hypertension, disappearance of the thrombus at the end of alteplase infusion coexisted with onset of haemodynamic stabilization. In a 70-year-old female, the thrombus, which persisted despite streptokinase administration, translocated suddenly (during echocardiography) to the pulmonary artery, which resulted in a deterioration in her status. After alteplase administration following heparin infusion, steady clinical and echocardiographic improvement was observed.