Blood clots occur commonly 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. These 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. Although factor V Leiden is a common genetic defect, the actual incidence of clots and emboli among people with the defect is unknown.