en Infectious Diseases Infectious Diseases Original Article Original Article Background: Inhibitory antibody to exogenous Factor VIII (FVIII) is a major complication of hemophilia treatment. This study was conducted to determine the prevalence of inhibitor antibody directed against FVIII.<br>Methods: From May 2010 to May 2011, 52 patients with severe hemophilia A admitted in Amirkola Children’s Hospital were evaluated. Those who had abnormal mixing study, antibody against FVIII were measured. Data were collected and analyzed. <br>Results: The age range of the patients was 4-60 years. The inhibitor antibody was seen in 9 (17.3%) patients. The mean age of patients with inhibitor at the time of diagnosis was 10.22 years (ranged 4-31 years). Old patients had more hemarthrosis than young patients. The mean level of inhibitor antibody was 8.47 Bethesda (ranged 2.3-29). Six patients had inhibitor antibody level ?5 Bethesda unit and three patients had inhibitor antibody level<5 Bethesda unit.<br>Conclusion: This study showed that the prevalence of inhibitor antibody in young patients is more than the old patients. <br> Inhibitory antibodies, concentrated FVIII, hemophilia A, Hemarthrosis 727 730 http://caspjim.com/browse.php?a_code=A-10-62-151&slc_lang=en&sid=1 Hassan Mahmoodi Nesheli 1003194753284600833 1003194753284600833 Yes Amereh Hadizadeh 1003194753284600834 1003194753284600834 No Ali Bijani 1003194753284600835 1003194753284600835 No