That factor XI has a role in normal blood coagulation is evidenced by the fact that patients with deficiency are prone to excessive bleeding after haemostatic challenge. The role of factor XI in physiological processes has become clearer since the discovery that it is activated by thrombin; this fact has contributed to a revised model of blood coagulation. Factor XI deficiency is particularly common in Ashkenazi Jews. Bleeding is typically provoked by surgery in areas of increased fibrinolysis, and is not restricted to individuals with severe deficiency. The bleeding tendency is variable and the reasons for this are not fully understood, although in severe deficiency there is some correlation between phenotype and genotype. The factor XI gene is 23 kb long, and two mutations are responsible for most factor XI deficiency in the Ashkenazi population. A total of 13 mutations have thus far been published. Factor XI deficient patients may need specific therapy to cover surgery and dental extractions. Although a factor XI concentrate is available there have been recent reports of coagulation activation and thrombosis indicating that it should be used cautiously. Fresh frozen plasma may be an acceptable alternative in some situations.