Development of factor VIII inhibitors remains the most serious and life-threatening complication of hemophilia therapy. The aim of this study was to determine the prevalence of inhibitor development in Turkish patients with hemophilia. Totally 1226 patients were screened [HA: 1057, HB: 105, von Willebrand's disease (vWD): 641. Ages ranged from 1 to 55 years (mean: 16.5 years). Sixty-two percent of patients (657/1057) were severe hemophilia. This study showed that inhibitor prevalence in Turkish hemophiliacs exposed to factor concentrates and fresh frozen plasma (FFP) is 11.2% for all HA and 15.8% for severe HA versus 1.9% for HB after eliminating transient inhibitors. Totally 122 patients were found inhibitor positive [high responder (HR) inhibitor= 60 and low responder (LR) inhibitor= 59 for HA/2 LR for HB/1 LR for vWD]. Thanks to this project, patients with inhibitor development can be treated with specific products such as recombinant factor VIIa or activated protrombin complex concentrates for their bleeding episodes or in their elective operations.