The purpose of this study was to determine the effect of ABO blood groups on von Willebrand factor-ristocetin cofactor activity (vWF-RCo) and on vWF-antigen (vWF-Ag) in children who have no personal or familial history of bleeding. Material and methods: A survey and testing were performed on 200 children with no personal or familial history of bleeding. In all, 100 of them belonged to blood group O, and the remaining 100 belonged to other blood groups. The blood samples were stored at -80 degrees C for a maximum period of 2 weeks to detect vWF-RCo and vWF-Ag levels. Results: The mean vWF-Ag (+/- 2 standard deviation [SD]) level in children with blood group O was 86% (+/- 20%); and for those with non-O blood group, it was 98.8% (+/- 25%). There was a significant difference between the 2 groups (P < .001). The mean vWF-RCo (+/- 2 SD) level in children with blood group O was 89% (+/- 23%); and for those with non-O blood group, it was 103% (+/- 17%). There was a significant difference between those in the 2 groups (P < .001). The lowest value of vWF-Ag and vWF-RCo levels in children with blood group O was found to be 50%. In conclusion, we showed that the selection of normal ranges based on the ABO group might influence the clinical diagnosis of vWD and that while the approach of using ABO group ranges for a vWF-Ag level lower than 50 IU/dL is scientifically sound, it might not be useful to assist a clinician in identifying people at increased risk of bleeding.