Most deep vein thromboses (DVTs) start in the calf, and most probably resolve spontaneously. Thrombi that remain confined to the calf rarely cause leg symptoms or symptomatic pulmonary embolism (PE). The probability that calf DVT will extend to involve the proximal veins and subsequently cause PE increases with the severity of the initiating prothrombotic stimulus. Although acute venous thromboembolism (VTE) usually presents with either leg or pulmonary symptoms, most patients have thrombosis at both sites at the time of diagnosis. Proximal DVTs resolve slowly during treatment with anticoagulants, and thrombi remain detectable in half of the patients after a year. Resolution of DVT is less likely in patients with a large initial thrombus or cancer. About 10% of patients with symptomatic DVTs develop severe post-thrombotic syndrome within 5 years, and recurrent ipsilateral DVT increases this risk. About 10% of PEs are rapidly fatal, and an additional 5% cause death later, despite diagnosis and treatment. About 50% of diagnosed PEs are associated with right ventricular dysfunction, which is associated with a approximately 5-fold greater in-hospital mortality. There is approximately 50% resolution of PE after 1 month of treatment, and perfusion eventually returns to normal in two thirds of patients. About 5% of treated patients with PE develop pulmonary hypertension as a result of poor resolution. After a course of treatment, the risk of recurrent thrombosis is higher (ie, approximately 10% per patient-year) in patients without reversible risk factors, in those with cancer, and in those with prothrombotic biochemical abnormalities such as antiphospholipid antibodies and homozygous factor V Leiden.