Doctors use blood thinners to help dissolve clots and prevent repeated clots. For many years, they most often used a blood-thinning drug known as unfractionated heparin(UFH). Patients get UFH through a needle in a vein for a few days while they are in the hospital. The dose of UFH is monitored closely each day to check the amount of blood thinning that occurs. More recently, doctors have been using another preparation of heparin, low-molecular-weight heparin (LMWH). It is injected under the skin, can be given at home, and does not require close monitoring of the dose. Several studies show that LMWH is a safe and effective drug for treating blood clots in the legs. However, we know less about the relative benefits of LMWH versus UFH in the treatment of patients with clots in their lungs.