We describe the case of a 57-year-old man with a mechanical aortic valve who had discontinued oral anticoagulants for a long period and presented with symptoms of severe congestive heart failure, including pulmonary oedema and hypotension. Clinical and echocardiographic examination revealed a large thrombus obstructing the prosthetic valve. Since his critical clinical condition was not improved by heparin treatment we decided to administer intravenous fibrinolysis with 100 mg rtPA. Fibrinolytic treatment was entirely successful, with complete thrombus dissolution and restoration of clinical status. Thrombosis of mechanical prosthetic valves is a particularly serious complication with a high mortality rate. Early diagnosis and treatment are of paramount importance for these patients. Although surgical intervention is the suggested treatment in many series, fibrinolytic therapy offers a good alternative with a high success rate, especially in the critically ill patient where surgery carries a high risk of mortality.