To investigate the increased tendency of hemorrhage in patients receiving valproate (VPA) therapy, we studied coagulation parameters in 30 randomized children of a group of 83 children receiving antiepileptic drug (AED) therapy. Besides a reduction in fibrinogen concentration and platelet count, we observed a significant decrease in factor VIII-complex. A decrease in factor VIII:C was noted in 33%, a decrease in von Willebrand factor (vWF:Ag) was noted in 83% and a decrease in ristocetin-cofactor activity (vWF:Rcof) was noted in 66% of the children. We classified a von Willebrand syndrome type I in 67% of our patients receiving VPA therapy. Sixty-three percent of patients had a history of bleeding, and 23% had a prolonged bleeding time. We compared our results with those of a control group and of a group of patients with congenital von Willebrand disease (vWD), from which patients with multimer types II and III were excluded. Because coagulation parameters in patients with congenital vWD are similar to those receiving AED therapy, we designated the increased tendency to hemorrhage as VPA-induced vWD. The decrease in coagulation parameters were not dependent on either VPA dose or period of administration. In patients receiving VPA therapy, this result must be considered, especially during surgical intervention and after traumatic events.