Abstract.We present the results of tests carried out to detect alloimmunization against red cells in 1,200 patients (607 males and 593 females), transfused and followed up during the period 1981–1987 in our hospital. Of these patients, 1,135 were thalassemic and 65 had sickle cell/β‐thalassemia. In 162 patients who Received: blood matched for the ABO, rhesus and Kell systems from their first transfusion, the immunization rate was very low (3.7%). In a pilot group consisting of 83 patients with the same clinical characteristics, who Received: blood matched only for the ABO and Rh‐D antigens, there was a significant difference in the frequency of alloantibodies (15.7%, p<0.001). Of 1,038 patients who Received: blood only matched for ABO and Rh‐D 244 (23.5%) with one or more red cell alloantibodies were identified. Of these 1,038 patients, 973 were exclusively thalassemic. In 220 (22.6%) of them, alloantibodies were found. The sickle cell β‐thalassemia patients presented alloantibodies with a higher frequency (36.9%, 24/65). Only one antibody was found in 114 patients (51.8%) and two or more in 106 patients (48.2%). The alloimmunization significantly concerned the rhesus (34.0%) and Kell (29.8%) systems. Anti‐Kell was most often identified (28.5%). Alloimmunization appears considerably lower in patients in whom blood transfusion is started before the age of 3 than in those in whom it is started after that age (20.9 vs. 47.