Red blood cell (RBC) transfusion is the primary treatment for severe forms of thalassaemia. Pre-storage screening has resulted in decreased transfusion-transmitted infections, but anti-RBC antibodies remain a major problem. We report on 697 participants who had ever received transfusions. Allo- and autoantibody rates were compared with respect to splenectomy status, ethnicity, diagnosis, duration of transfusions, treatment centre, and age at transfusion initiation, together with rates before and after 1990, when leucoreduction methods were routine at thalassaemia treatment centres. Allo- and autoantibodies were reported in 115 (16·5%) and 34 (4·9%) subjects, respectively. Splenectomized patients were more likely to have alloantibodies [odds ratio (OR) = 2·528, P ≤ 0·0001], or autoantibodies (OR = 2·590, P = 0·0133). Alloantibodies occurred in 19 of 91 (21%) splenectomized subjects who started transfusion after 1990, and only 18 of 233 (7·7%) nonsplenectomized subjects (P < 0·001). Data from this study demonstrate that RBC antibodies continue to develop in chronically transfused thalassaemia patients at a high rate. Splenectomy preceded the development of antibodies in most cases. Increased rates of RBC sensitization among splenectomized patients is concerning and deserves further study.