In this study, we aimed to determine the frequency of red cell allo- and autoimmunization and analyze the factors responsible for the development of antibodies in patients with transfusion-dependent thalassemia. This crosssectional study was conducted on 139 patients with thalassemia major and intermedia who received leukodepleted RBC transfusions on a regular basis. Patients with a positive antibody screen were further tested for antibody identification by a gel method. Red cell alloantibodies were found in 9 (6.4%) patients, and autoantibodies were found in 17 (12.2%) patients. The most common alloantibodies detected were those against Rh and Kell antigen systems. The alloantibody development rate was higher in thalassemia intermedia patients, in Rh(-) patients, in patients with an initial transfusion age >2 years and in patients with a transfusion interval >3 weeks (p<0.05). The autoantibody development rate was found to be higher in adult and splenectomized patients (p<0.05). Data from this study demonstrate that the RBC antibody development rate is high in our region. RBC antigen phenotyping and crossmatching with Kell and Rh subgroups may reduce alloimmunization in chronically transfused beta-thalassemia patients.