Factor XI deficiency, also called hemophilia C, was first described in 1953. It is thought to constitute 7% of all disorders of the intrinsic pathway of blood coagulation. It represents the only deficiency of a contact phase protein associated with a bleeding tendency with widely variable expression. Eighteen members of 5 families with this deficiency have been investigated; only 3 exhibited a hemorrhagic diathesis: 2/4 homozygous (hemorrhages after surgical procedures and hematomas and after minor traumatism) and 1/10 heterozygous individuals (hematomas, epistaxis). All homozygotes had a prolongation of the PTT. The hemorrhagic tendency was higher among homozygous individuals (with very low factor XI activity), but 2 were asymptomatic. Heterozygotes may also bleed. This deficiency is found mainly in an Ashkenazy Jewish population. Our study shows that it occurs in an apparently autochthonous Swiss population. A detailed history and studies of other family members are necessary if there is suspicion of factor XI deficiency.