A bleeding tendency is a frequent cause of hematological consultation. However, despite extensive biological investigations, no hemostasis abnormality is identified in the majority of patients, leading to the diagnosis of bleeding of unknown cause (BUC). Overall, the pathogenesis of BUC is most likely of multifactorial etiology and the accumulation of minor alterations in fibrin clot structure, fibrinolysis, platelet function and thrombin generation can tip the balance of hemostasis towards the bleeding side. In recent years, unconventional biological explorations have found new hemorrhagic abnormalities. Therapeutic options are limited but are usually effective to prevent bleeding.