Purpose: We performed this study to investigate the incidence and clinical significance of acquired von Willebrand syndrome (AVWS) in adult patients treated with extracorporeal membrane oxygenation (ECMO). Methods: We reviewed medical records of adult patients (age ≥ 19 years) admitted to medical intensive care unit for ECMO support between January 2019 and June 2019. The AVWS was defined as the ratio of ristocetin cofactor activity to von Willebrand factor antigen (vWF:RCo/vWF:Ag) < 0.7 or any abnormality in von Willebrand factor multimer analysis. Clinical characteristics and outcomes were compared between AVWS and non-AVWS groups. Results: During the study period, 25 patients had received ECMO support and 19 (76.0%) patients had completed the laboratory tests for AVWS. Among 19 patients included, median age was 63 years (interquartile ranges [IQR]: 54-65) and 13 (68.4%) were male. Median duration of ECMO support was 6 days (IQR: 3-18) and 14 (73.7%) were supported with veno-venous configuration. The AVWS was diagnosed in 12 (63.2%) patients. The incidence of hemorrhagic complication was significantly higher in AVWS group (91.7% [n=11/12] in AVWS group and 42.9% [n=3/7] in non-AVWS group, p=0.038). However, there were no significant differences in other clinical outcomes including intensive care unit mortality (25.0% [n=3/12] in AVWS group and 28.6% [n=2/7] in non-AVWS group, p >0.999) and hospital mortality (33.3% [n=4/12] in AVWS group and 42.9% [n=3/7] in non-AVWS group, p>0.999). Conclusions: AVWS is common but under-recognized in adult patients treated with ECMO. The AVWS seems to be associated with hemorrhagic complications during ECMO support, but its significance on mortality is unclear.