: Congenital factor XIII (FXIII) deficiency is a rare bleeding disorder accompanied by a variety of bleeding events. Severely deficient patients require regular replacement therapy. With development of FXIII concentrate, the risk of viral infections transmitted by fresh frozen plasma and cryoprecipitate is diminished, but the possibility of inhibitor development remains a challenging issue in the management of these patients. The aim of this study was to assess FXIII inhibitor development in Iranian patients with FXIII deficiency (FXIIID). This study enrolled 50 (30 women and 20 men) patients with severe congenital FXIIID from southeast Iran who underwent long-term (more than 4 years or more than 50 injections) prophylaxis with FXIII concentrate (Fibrogammin P, Dade Behring, Marburg, Germany). We evaluated plasma FXIII activity and FXIII inhibitor on day 28 after the last prophylaxis administration. The method for investigation of FXIII inhibitor was based on Bethesda assay. The mean age of the study population was 13.8 ± 8.3 years. The minimum and maximum FXIII activity levels were less than 1-4.5% (mean ± SD, 2.6 ± 0.7%). Our investigations showed that all patients with severe form of FXIIID were treated without inciting inhibitor development. Despite long-term prophylaxis in the studied patients, none was found to have developed FXIII inhibitors.