 Heparin is the reference therapy for most patients with pulmonary embolism. Some patients with sub-massive pulmonary embolism defined by normal blood pressure and dysfunction of the right ventricle have a higher mortality risk. It has been suggested that thrombolytic treatment, a drug that dissolves blood clots more rapidly, may reduce the mortality in those patients. The studies reported to date were unable to confirm or refute this hypothesis because the number of patients included in those studies is too low. The aim of the study is to compare thrombolytic treatment with heparin which is the reference therapy for pulmonary embolism in a large group of patients with sub-massive pulmonary embolism. 