Also called: Christmas disease, Factor IX deficiency, Factor VIII deficiency

Hemophilia is a rare inherited disorder in which the blood does not clot normally. About 18,000 people in the U.S. have hemophilia. Each year, about 400 babies are born with the disorder. Hemophilia usually occurs in males (with very rare exceptions). People who have hemophilia may bleed for a long time after an injury or accident. They also may bleed into their knees, ankles and elbows. Bleeding in the joints causes pain and, if not treated, can lead to arthritis. Bleeding in the brain, a very serious complication of hemophilia, requires emergency treatment. The main treatment is injecting the missing clotting factor into the bloodstream.