Background: Blood transfusion is an integral part of the supportive care for patients with sickle cell disease (SCD) and thalassaemia. The hazard of red cell alloimmunization, however, is one of the main complications of this therapy. Objectives: The aim of this study was to evaluate the prevalence of red cell alloimmunization in Omani patients with sickle cell anaemia and thalassemia. Methods: This study included 262 patients whose historical transfusion records were available. One hundred and twenty-nine patients with thalassaemia who were attending the day care unit for regular transfusions, and 133 SCD patients admitted at our hospital were included in this study. The Diamed® gel system was used for the screening and identification of atypical antibodies. Results: The rate of alloimmunization in SCD patients was 31.6% (n=42, 95%CI, 24.87-40.66), whereas in patients with thalassaemia it was 20% (n=26; 95%CI, 13.9-27.6). Antibodies to E, e, C, c, D, K, S, Fyª, Kpª, Jkª and Cw were observed; 85% of the patients were also immunised with Rh and Kell antigens. Considering the two groups together, 8 developed nonspecific antibodies and 12 developed more than one antibody. Conclusions: Red cell transfusions were associated with a significant risk of alloimmunization. It is, therefore, imperative to perform an initial extended red cell phenotyping for both donors and recipients, and carefully select ABO, Rh and Kell matched donors. The higher incidence of alloimmunization in SCD patients is related to the inherent SCD-specific inflammatory state.