Individuals with Type O blood have been reported to have a tendency toward reduced Factor VIII and von Willebrand Factor (vWF) levels. If this is true, patients with Type O blood might be vulnerable to coagulopathy during acute normovolemic hemodilution using hydroxyethyl starch (HES), both from hemodilution as well as HES-related coagulopathy. Thirty non-O and 15 type O ASA 1 or 2 patients scheduled for spinal surgery involving more than two spinal levels were enrolled for the study. After anesthesia induction, 30% of the estimated blood volume was removed, and the volume was simultaneously replaced with 6% HES (130/0.4). Coagulation profiles were measured before (T0) and 30 min after acute normovolemic hemodilution (T30). Factor VIII activity, vWF antigen levels (vWF:ag), and vWF ristocetin cofactor activity (vWF:RCof) were lower in the O group than in the non-O group before and after acute normovolemic hemodilution, and decreased below the normal range in the O group after acute normovolemic hemodilution. The decrease was beyond that expected from hemodilution alone. Maximum amplitude and coagulation index of the thromboelastogram decreased below the normal range in the O group after acute normovolemic hemodilution. The decrease in vWF:ag was related to the degree of blood loss, and was greater in patients in the O group. Patients with Type O blood may have increased coagulation compromise, and greater dilution of Factor VIII activity, vWF:ag, and vWF:RCof after acute normovolemic hemodilution with HES.