Acquired von Willebrand factor (vWF) disease is associated with a decrease in the amount of circulating high molecular weight (HMW) vWF multimers. vWF has not been previously investigated in children on extracorporeal membrane oxygenation (ECMO) support. We hypothesized that HMW vWF multimers and vWF activity decrease over the course of ECMO support in these patients. This prospective, single center, observational, cohort pilot study was carried out between December 2010 and April 2011 and included patients 0 to 18 years old requiring ECMO support at our institution. Blood samples were tested for various aspects of vWF. Mean and standard deviation were estimated for vWF activity and multimers, whereas a generalized linear model was developed to estimate multimer changes over time.The study included six pediatric patients. The mean age of the patients was 54.9 ± 55.3 (mean ± standard deviation) months. The mean HMW vWF multimer percentage was 23.4 ± 7.3 in the pre-ECMO samples and significantly decreased over time (p<0.003). There was no significant change in low molecular weight vWF multimer percentage. An immediate decrease in vWF HMW multimers as a percentage of all multimers once ECMO is initiated was noted and persisted across the study period.