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Although two clusters have been identified in France, constitutional factor XI deficiency is a rare disorder. Acquired factor XI deficiency is extremely rare. The management of factor XI deficiency is not straightforward because of the unpredictable bleeding tendency that does not clearly relate to the factor XI level. Other hemostasis parameters must be taken into account to evaluate the bleeding tendency. We report the cases of a congenital factor XI deficiency, an acquired factor XI deficiency, and a von Willebrand disease associated with a factor XI deficiency. On the other hand, some interferences can lead to underestimation of factor XI, and we report the case of an interference by a lupus-type circulating anticoagulant. The objective of this review is to better understand how to manage a reduced factor XI level.