Veno-arterial extracorporeal membrane oxygenation (VA ECMO) is a powerful device for treatment of patients with life-threatening heart failure. Although bleeding is often associated with VA ECMO and sometimes results in a fatal outcome, its precise causes remain unknown. On the other hand, excessive high shear stress in the cardiovascular system causes acquired von Willebrand syndrome (aVWS), characterized by loss of von Willebrand factor (vWF) large multimers. vWF large multimers of five consecutive patients treated with VA ECMO were quantitatively evaluated using the vWF large multimer indices, defined as the ratio of the large multimer ratio of a patient to that of a healthy subject analyzed simultaneously. All 5 patients exhibited oozing type of bleeding at the skin insertion sites under treatment with PCPS at flow rates of 2.5–3.0 l/min/m², including two severe cases of bleeding; one patient had massive gastrointestinal bleeding and another had hemothorax. Their vWF large multimer indices were 20.8, 28.8, 27.6, 51.0, and 31.0% (means 31.8 ± 11.4%). Surprisingly, these values are much lower than those observed in severe aortic stenosis reported previously by us (Tamura et al. in J Atheroscler Thromb 22:1115–1123, 2015), where vWF multimer indices in 31 severe aortic stenosis patients with peak pressure gradient through the aortic valves of 85.1 ± 29.4 mmHg were 75.0 ± 21.7% (p < 0.0001), indicating that much higher grade of aVWS occurred in patients with VA ECMO than severe aortic stenosis patients. All the 5 patients treated with VA ECMO developed aVWS that was much more severe than in patients with severe aortic stenosis.