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Individuals with factor XI (FXI) deficiency have a variable bleeding risk that cannot be predicted from plasma FXI antigen or activity. This limitation can result in under- or overtreatment of patients and risk of bleeding or thrombosis. Previously, plasma clot fibrinolysis assays showed sensitivity to bleeding tendency in a small cohort of patients with severe FXI deficiency. Here, we determined the ability of plasma clot formation, structure, and fibrinolysis assays to predict bleeding tendency in a larger, independent cohort of patients with severe and partial FXI deficiency. Patients were characterized as nonbleeders or bleeders based on bleeding after tonsillectomy and/or dental extraction before diagnosis of FXI deficiency. Blood was collected in the absence or presence of the contact pathway inhibitor corn trypsin inhibitor (CTI). Clotting was triggered in platelet-poor plasma with tissue factor, CaCl 2 , and phospholipids in the absence and presence of thrombomodulin or tissue plasminogen activator. Clot formation and fibrinolysis were assessed by turbidity and confocal microscopy. CTI-treated plasmas from bleeders showed significantly reduced clot formation and decreased resistance to fibrinolysis compared with plasmas from controls or nonbleeders. Differences were enhanced in the presence of CTI. A model that combines activated partial thromboplastin time with the rate of clot formation and area under the curve in fibrinolysis assays identifies most FXI-deficient bleeders. These results show assays with CTI-treated platelet-poor plasma reveal clotting and clot stability deficiencies that are highly associated with bleeding tendency. Turbidity-based fibrinolysis assays may have clinical utility for predicting bleeding risk in patients with severe or partial FXI deficiency., (© 2018 by The American Society of Hematology.)