The rate of alloimmunization to red blood cell antigens in 1502 multitransfused patients, mainly with blood disorders, was analyzed in a retrospective study. The overall incidence of alloantibodies was 5.7%. Three groups of patients were identified with different potential for antibody production. The lowest probability (1.8%) of alloimmunization was found in the group of patients with lymphoproliferative syndromes, acute myeloid leukaemia and burn disease. The highest probability (33.4%) of immune response to red blood cell antigens was found in patients with AIHA, liver cirrhosis and myelodysplastic syndrome. In the group of patients with chronic myeloid leukaemia, pancytopenias, anaemias of various origin and aplastic anaemia the probability of alloimmunization ranged from 5.7% to 13.6%. A possible role of genetic-factors and immune competence status in post-transfusion alloimmunization is briefly discussed.