A thrombotic tendency (venous or arterial) has been reported in some cases of dysfibrinogenemia. We report here the mechanism by which these thrombosis may occur. It may be related either to a defective clot lysis due to a poor reactivity toward fibrinolytic enzymes or to a defective thrombin binding capacity of the abnormal clot. Acquired fibrin clot structure anomalies may also be responsible for a defective thrombolysis.