One hundred sixty-four previously transfused patients with T/H disease received one to 136 transfusions with an average of 14.65. Immunization to red blood cell occurred in 14 patients (8.5%). Among these, 11 patients had alloantibody, one patient had autoimmune antibody, and two patients had both alloantibody and autoimmune antibody. The naturally occurring antibody (anti-Leb) was observed in pretransfused blood samples of two patients (1.2%). Most red cell immunization seemed to occur in the early period of blood transfusion.