Massachusetts Eye & Ear Infirmary

Hemophilia is an inherited bleeding disorder in which the blood does not clot normally because of a missing or dysfunctional clotting protein. Clotting proteins, also called clotting factors, work with other blood components such as platelets, von Willebrand factor, and other proteins to form a blood clot. When any of these components is absent or low, increased bleeding can result. If clotting factor VIII (8) or factor IX (9) are affected, this bleeding disorder is called hemophilia.