Factor XI (FXI) deficiency is an autosomal bleeding disorder characterized by variable bleeding tendency. In the present study, the gene encoding FXI (F11) was analyzed by direct sequencing in 33 individuals belonging to 11 unrelated Turkish families, and the bleeding tendency was quantitatively assessed by means of a bleeding questionnaire in 27 individuals with low FXI clotting activity and/or mutated F11 gene. We identified 10 distinct mutations (five missense, three nonsense and two splice site), four of which were novel. No mutation was found in one family. Of the four novel mutations, homozygosity for a c.89T>C (p.Phe30Ser) mutation and compound heterozygosity for a c.646G>A (p.Asp216Asn) mutation with the known c.403G>T (p.Glu135) type II Jewish mutation were associated with severe deficiency, whilst heterozygosity for the novel c.1655A>C (p.His552Arg) and c.1627G>A (p.Glu543Lys) mutations was associated with partial deficiency. p.Glu135 was found in 19% (5/27) of the mutated alleles. Bleeding score was positive in 57% (4/7) of individuals with severe and 39% (7/18) of those with partial deficiency. It was significantly correlated with clinical severity of bleeding (r=0.43, P=0.02), but not with FXI clotting activity (P>0.05). There was no optimal cut-off level of the bleeding score that could predict FXI deficiency. We conclude that the spectrum of mutations found in this study reflects the genetic heterogeneity of FXI deficiency in the Turkish population. Quantitative assessment of the bleeding symptoms by a bleeding questionnaire seems to be useful for evaluating the severity of bleeding episodes, but it can not be recommended as a screening tool for FXI deficiency. (C) 2015 Wolters Kluwer Health, Inc. All rights reserved.