Deep venous thrombosis (DVT) is a condition in which blood clots form in the large leg veins, causing pain and swelling. It is important to treat DVT because clots can break off and travel through the bloodstream to the lungs, a potentially deadly condition called pulmonary embolism. DVT is most commonly treated with blood thinners, usually starting with standard heparin given intravenously (through a small tube inserted into a vein) for 5 to 10 days, followed by warfarin, a pill that is generally taken for at least 3 months. A new type of blood thinner called low-molecular-weight heparin has recently become available. Treatment with low-molecular-weight heparin is more convenient since it can simply be injected under the skin once or twice a day. In addition, unlike patients receiving standard heparin, patients receiving low-molecular-weight heparin do not need frequent blood tests to guide dose changes. Several studies have suggested that low-molecular-weight heparin is an effective treatment for DVT. However, those studies were small, did not include patients who had DVT plus pulmonary embolism, and used two injections per day rather than one.