Blood clots can form in the deep veins of the leg, a condition called “deep venous thrombosis," as a complication of hip or knee replacement surgery. These clots can break off and travel through the bloodstream to the lungs, shutting off some of the lung's circulation, a condition called “pulmonary embolism.” Giving patients blood thinners in the hospital after joint replacement surgery can prevent deep venous thrombosis and pulmonary embolism. However, it remains unknown how long the blood thinners should be continued. Commonly used blood thinners are standard heparin (usually given through an intravenous line), warfarin (given by mouth), or low-molecular-weight heparin (a new form of the drug given by injection under the skin). Low-molecular-weight heparin is more convenient than regular heparin because it can be given once daily and, unlike heparin and warfarin, does not require frequent blood tests to adjust the dose.