Circulating anticoagulants are endogenous blood components that inhibit the action of clotting factors. In some inhibitor conditions this inactivation in the function of the hemostatic system may lead to life-threatening hemorrhagic diathesis. Inhibitors directed against factor XI are generally associated with little or no impairment of the hemostatic system. We analyzed all reported cases of spontaneous factor XI inhibitor in the international literature, as well as cases identified at the Yale--New Haven (Conn) Hospital between 1970 and 1980, considering clinical spectrum, diagnosis, and therapy.