OBJECTIVE To investigate the seroprevalence and specificity of red blood cell (RBC) antibodies in multitransfused patients, in whom the risk of alloimmunization is especially high. METHODS We conducted a retrospective study on blood specimens from 200 multitransfused patients. We evaluated all specimens for alloimmunization using various immunohematological tests via the column agglutination technique. RESULTS The overall prevalence of RBC alloantibodies was 5.5%. Of the 11 specific types of alloantibodies identified, most (72.7%) belonged to the Rh blood group system, followed by the S, M, and Lewis blood group systems (9.1% each). CONCLUSIONS Most alloantibodies were of the Rh blood group specificity. To improve the quality of blood supplied, especially to patients with thalassemia, we recommend that Rh phenotyped, cross-match-compatible blood should be issued to prevent complications such as acute and delayed hemolytic reactions.