Severe factor XI (FXI) deficiency is an injury-related bleeding disorder common in Ashkenazi Jews and rare worldwide. In the past two decades, more than 180 mutations in the FXI gene have been reported in patients with FXI deficiency, five of which show a founder effect (Cys38Arg, Gln88Stop, Cys128Stop, Glu117stop, and Phe283Leu, the last two largely prevalent among Ashkenazi Jews). Inhibitors to FXI after exposure to plasma, FXI concentrates, or Rh immunoglobulin were described in patients with mutations resulting in null alleles. Treatment with low-dose recombinant activated factor VII in these patients appears promising. Survival advantages to patients with severe FXI have been recently reported. Herein, we present new observations related to clinic presentation, genotype-phenotype correlation, and treatment problems in patients with FXI deficiency.