Deep venous thrombosis (DVT), a condition in which blood clots form in leg veins and cause pain and swelling, is a common complication of knee replacement surgery. Pieces of these clots can break off and travel to the lungs, causing a serious and potentially deadly condition called pulmonary embolism. To prevent DVT, doctors prescribe blood thinners around the time of knee replacement surgery. Warfarin is a common blood thinner used for this purpose. Unfortunately, up to almost half of patients still develop DVT after knee replacement surgery even if they take warfarin. In addition, warfarin requires frequent blood tests and dose changes to make sure that patients are getting an appropriate amount of the drug. Ximelagatran is a newer blood thinner that has shown some promise in preventing DVT after hip and knee replacement surgery. An advantage of ximelagatran is that it does not require blood tests or dose changes.