Four factor XI (F XI)-deficient patients are described, all of whom formed circulating anticoagulants against F X1. In the three most severely affected patients (F XI 0%-6% activity), the anticoagulant appeared to have been stimulated by plasma infusion. However, in the milder case (25% F XI activity), no infusion had been documented. The findings in these cases emphasize the diversity of F XI inhibitors in congenitally deficient patients. Awareness of the potential development of these inhibitors will be helpful in both daily management and perioperative care of such patients.