The spontaneous onset of autoantibodies against circulating factors, including factors II, V, VII, VIII, IX, XI, XII, and XIII, is a phenomenon of unclear causes. It may occur in patients with no underlying disease process or may be antedated by autoimmune diseases and malignancies. The antibodies are almost always IgG and are most commonly directed at F VIII. They generally present with a bleeding diathesis in patients with no history of prior bleeding events. The laboratory abnormalities vary depending on whether the inhibitor involves the intrinsic or extrinsic pathway of coagulation, and the laboratory studies may vary depending on the technique used. Supportive measures are used initially but are generally not completely effective. Supportive measures are followed by blood products, such as recombinant human F VIII, porcine F VIII, or PCC/aPCC. There are limitations to each of these, however. If bleeding is severe and high titers of inhibitors are present, plasma-pheresis may lead to a temporary decrease in the titer but is short-lived. Immunosuppressive therapy should ultimately be attempted to suppress further antibody production. Spontaneous remission may occur with some of the inhibitors, primarily postpartum or drug related, for reasons not fully understood.