Mobile right atrial thrombi carry a high risk of pulmonary embolism which may be massive and are a medical emergency. Although surgery is commonly indicated, treatment with intravenous thrombolytics is an alternative and was successful in 4 out of 6 cases reported by the authors. Six patients, admitted for severe pulmonary embolism confirmed by pulmonary scintigraphy (6 cases) and by angiography (2 cases), underwent echocardiography which demonstrated a mobile right atrial thrombus. One patient was operated as an emergency and died immediately afterwards. Another, treated with heparin because of contraindications to surgery and thrombolysis had a recurrent fatal pulmonary embolism. In the other four cases, intravenous thrombolytic therapy was started immediately after echocardiography with 250,000 IU of streptokinase in 30 minutes, followed by 100,000 IU per hour for 48 to 72 hours associated with heparin 300 to 500 IU/kg/day. The biological efficacy of the treatment was confirmed in all cases (fibrinogen < 1 milligram; TCA > 60 s). A clinical improvement with improved blood gases was rapidly obtained in all 4 cases. The thrombus had totally disappeared at control echocardiography 8 to 12 hours after the initial examination. There were no complications, in particular no haemorrhages. After 6 months' follow-up, the outcome was good with oral anticoagulants (4 cases) associated with implantation of a caval filter in 1 case. Thrombolysis seems to be an effective alternative to surgery as there four cases demonstrate.