The development of erythrocyte alloantibodies complicates transfusion therapy in patients with thalassemia. However, no data are available on the frequency of erythrocyte alloimmunization in patients with transfusion-dependent thalassemia in Taiwan. We analysed the clinical and transfusion records of 30 individuals (15 females and 15 males; mean age, 20 years; range, 4-31 years) with thalassemia who had regular transfusions for periods ranging from 0.5 to 20 years. Of the 30 patients, 28 who had beta-thalassemia major and two who had Hb H disease (alpha thalassemia), 11 (37%) were found to carry alloantibodies. All alloantibodies were clinically significant specificities, including four cases of anti-E, two of anti-E + c, two of anti-'Mi(a)', one of anti-'Mi(a)' + E, one of anti-D and one of anti-S. Alloimmunization to erythrocyte antigens is a frequent complication in transfusion-dependent thalassemia.