Present evidence suggests that venous thromboembolism is the third most common acute cardiovascular disease after cardiac ischemic syndromes and stroke. The frequency of the diagnosis of pulmonary embolism (PE) at a given hospital greatly increases if a referral unit for PE is set up in the hospital. Pulmonary embolism is characterized by a continuous spectrum of severity, from 2 to 3 to 15 to 16 embolized pulmonary segments (over a total of 19). Morbidity from PE increases with age and male sex (males/females ratio: 1.24). In only a minority (10%) of patients with PE and/or deep-vein thrombosis (DVT), primary deficiencies of coagulation-inhibiting proteins have been shown. Primary abnormalities of the fibrinolytic system seem even more rare. On the basis of the clinical conditions preceding the embolic episode, patients may be divided into different groups: apparently primary or idiopathic PE (40%), surgery or trauma (43%), heart disease (12%), neoplastic disease (4%), and systemic disease (1%). Patients with apparently primary or idiopathic PE often develop subsequent clinically overt cancer (9.1%), whereas surgery or trauma patients rarely do (1.4%). Furthermore, the former exhibit a significantly shorter survival than the latter mostly for causes of death that reflect increased predisposition to thrombogenesis. Thus, as for DVT, it is convenient to consider a primary or idiopathic form also for PE.