Bleeding risk is not predictable in patients with factor XI (FXI; F11) deficiency. In this prospective study, our objectives were to determine the biological determinants for bleeding risk in patients with heterozygous FXI deficiency. Patients were classified as either bleeding patients or non-bleeding patients by calculating the bleeding score (BS) described for von Willebrand disease. Primary haemostasis, thrombin generation, thromboelastometry, procoagulant proteins, inhibitors, fibrinolysis, and F11 gene mutations were compared between bleeding and non-bleeding patients. Thirty-nine patients were included. BS significantly correlated with clinical assessment (P=0*001), and a score over 3 discriminated between bleeding (n=15) and non-bleeding (n=24) patients (P=0*034). Despite normal values, von Willebrand factor (VWF) and thrombomodulin (TM) plasma levels were significantly lower in bleeding patients than non-bleeding patients [ristocetin cofactor activity (VWF:RCo)=80*6±29*7 iu/dl and 101*8±29*5iu/dl respectively, P=0*043; and VWF antigen (VWF:Ag)=84*0±28*0 iu/dl and 106*3±36*1 iu/dl respectively, P=0*035; and TM=17*7±11*7ng/ml and 23*6±9*7ng/ml respectively, P=0*043]. When considering BS as a continuous variable, only VWF:RCo remained significant (P=0*042), which accounted for 11% of the variability in BS.