In patients with major acute PE thrombolysis has been shown to be life saving (22). Recent registries showed the beneficial effect of thrombolysis also in patients with PE not associated with shock or hypotension (4-23). In these patients thrombolytic treatment has been shown to obtain an improvement of 37% in lung perfusion, detected by lung scan, with respect to an improvement of 18.8% obtained by heparin treatment (24). In a different trial, rt-PA resulted in a faster and greater improvement of pulmonary artery hypertension than heparin treatment (25). Indeed, PE has a wide spectrum of severity at presentation and it is conceivable that the use of more aggressive treatments should be reserved to patients at high risk for adverse outcome. Hence, the search started of prognostic factors of adverse outcome in patients with pulmonary embolism.