PURPOSE: Antibody formation against red blood cells' antigen is a very important complication due to transfusions, and it can make the following transfusions difficult. To avoid this, it has been proposed giving identical red blood cells for the antigens more frequently involved in sensitizations. To evaluate this fact, we have accomplished a study with the transfused patients in our Hospital since 1990. MATERIAL AND METHODS: 10,308 transfused patients in the Hospital Nitra. Sra. de Covadonga (Oviedo), between January 1990 and March 1994, were studied. The patients have been included in two groups: The first one was constituted by 4,226 patients from the Haematology and the Nephrology Departments, who received red blood cells units compatible with ABO and CcDEe antigens. The second group was formed by the remainding patients transfused with red blood cells compatible only with ABO and D antigens. RESULTS: All 165 antibodies were detected in 132 patients, which means an incidence of 1.3 percent. In 63 cases, antibodies were present before the first transfusion. In the remaining patients, an allosentitization of 0.2 percent in group 1 and 1 percent in group 2 (p < 0.0001) was shown. This difference cannot be explained only for transfusing red blood cells with the same Rh phenotype in the group 1, because a lower immune response had persisted when we analyzed the other antibodies. More than 70 percent of antibodies appeared before the 10th transfusion. DISCUSSION: A lower sensitization exist in the patients of group 1. This seems to be caused by a state of immunosuppression for their disease or their treatments. However, in some patients, the risk of allosensitization persists, so we think it is a good practice to transfuse red blood cells without the most immunogenic antigens in haematological and nephrological patients who already have one antibody.