The present study was carried out to determine the evidence of alloimmunization against red blood cells in 364 patients transfused in our center over a period of 4 years (1990-1993). Among these patients, 127 were thalassemic and 182 had sickle cell disease (SCD). In 55 control patients, who received blood matched for the ABO, Rhesus and Kell antigen systems from the outset of transfusion, no immunization was detected. However, in the study group, who initially received blood matched only for ABH and Rh D antigens, the frequency of alloimmunization was 7.76% (24/309). Only one antibody was detected in 15 patients (62.5%) and two or more in 9 patients (37.5%). Alloimmunization concerned the Rhesus system in 58.82% of cases and the Kell system in 26.47%, while the frequency of immunization was significantly lower in patients of less than 5 years as compared to those in the age range 5-10 years (p < 0.001).