Alloimmunization to red blood cell (RBC) antigens may cause a delayed hemolytic transfusion reactions (DHTR) and a delayed serologic transfusion reactions (DSTR). In the present study, the frequency of alloimmunization and its clinical significance were evaluated. Also, transfusions were correlated with antibody formation. Alloimmunization rate was 0.63%. Alloimmunization rate in multiple transfused patients was 24.5%. The most common clinically significant alloantibodies of alloimmunized patients were found to be Rh antibodies (52.6%). Nine patients out of 38 (23.7%) became undetectable after the first detection. To be positive at antibody screening test after RBC transfusion was mean transfused numbers: 3.7 units, mean transfused periods: 56 days, mean transfused frequencies: 1.7 times. The results from antibody specificity and RBC transfusions were comparatively analyzed and it shows that Rh system antibodies were longer than other antibodies (P<0.05). In case of disease group, malignant diseases was longer than other diseases (P<0.05). In order to prevent the formation of RBC alloimmunization, irregular antibody screening tests were performed at propriety intervals in multiple transfused patients.