Blood clots sometimes occur in deep veins in the legs. Pieces of these clots, known as emboli, can break off and travel through the bloodstream, blocking arteries in the lungs. The blood clots and emboli can cause serious symptoms and death. Surgery, pregnancy, trauma, hormone therapy, and inherited blood disorders increase risks for blood clots and emboli. Factor V Leiden is one of these inherited disorders. Some studies show that it is associated with very high risks for clots. However, many researchers wonder about the actual size or amount of risks for clots and emboli among people with the defect.