The few inhibitors to blood coagulation factor IX in patients with Christmas disease (hemophilia B) that have been studied have been shown to belong to the IgG class of immunoglobulins. We report the first case in which a factor IX inhibitor was of the IgA class. Additionally, he appears to be the youngest patient with hemophilia B to have developed an inhibitor. His inhibitor complicated treatment of the patient for several years because of its anamnestic rise following factor IX concentrate administration. It then spontaneously vanished and has not returned in spite of repeated factor IX complex administration.