Congenital factor XI deficiency is a very rare blood-clotting disorder present at birth that results in excessive or prolonged bleeding after an injury or surgery. Factor XI is one of 13 proteins involved in proper formation of blood clots. Blood clots are needed to heal wounds, form scabs, and stop bleeding. When factor XI levels are low or absent, the blood does not clot correctly, leading to excessive bleeding. Congenital factor XI deficiency runs in families and is more common in the Jewish population, especially Ashkenazy Jews. The main symptom of congenital factor XI deficiency is excessive and abnormal bleeding. This may occur after childbirth, surgery, trauma, and with menstruation (periods). Bleeding can also occur in the muscles, joints, the mouth, the gut, or, infrequently, the brain. Easy bruising and nosebleeds are also common. Congenital factor XI deficiency can be diagnosed by a physician using blood tests. Treatment for congenital factor XI deficiency is largely based on controlling bleeding and treating any underlying conditions that contribute to excessive bleeding. When necessary, excessive bleeding can be stopped with infusions of clotting factors into the blood.