Materials and Methods Patient Population Out-patients presenting with suspected acute PE, when diagnostic imaging were not immediately available (during nights or week-ends), will be considered eligible for the study. The study will be conducted in three Institutions (University Hospital AOUP “Paolo Giaccone” of Palermo, IRCCS Policlinico S. Matteo, Italy and IRCCS Ospedale Maggiore di Milano, Italy) with the same protocol. Exclusion criteria included the following: 1) life-threatening conditions, or serious co-morbidities that required immediate hospitalisation ; 2) relapse of a previously documented PE episode; 3) current use of oral anticoagulant therapy; 4) history of bleeding or any other contraindication to heparin; 5) age younger than 18 years; 6) life expectancy of less than 3 months; 7) refusal to give informed consent.