Most deep vein thromboses (DVTs) start in the calf; however, thrombi that remain confined to the calf rarely cause leg symptoms or are associated with 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 and if this stimulus persists. 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. Treated proximal DVTs resolve slowly, and half of patients still have detectable thrombi after a year. About 10% of patients with symptomatic DVT develop severe postthrombotic syndrome within 5 years. This is more likely to occur if there has been an ipsilateral recurrent DVT. About 10% of PEs are rapidly fatal. Of PEs that are diagnosed before death, about 50% are associated right ventricular dysfunction, a finding that is associated with a high short-term mortality. There is about 50% resolution of PE after 1 month of treatment, and perfusion eventually returns to normal in two thirds of patients. After a course of treatment, the risk of recurrent thrombosis is higher in patients without a reversible risk factor and in those with certain biochemical abnormalities, including antiphospholipid antibodies, hyperhomocysteinemia, and homozygous factor V Leiden.