Allogeneic bone marrow transplantation (BMT) is an effective treatment for some severe hematologic or nonhematologic diseases. The blood group antigen mismatch between donor and recipient may cause immunohematological complications during or after BMT. In this review, we analyze the ABO, Rh and other red cell antigen mismatches between donor and recipient, the main immunohematological complications and the techniques to prevent them. The data reported are derived from the experience of the authors and from the medical literature. The clinical implications of the immunohematological aspects of BMT emphasize the importance of close immunohematological monitoring in patients undergoing allogeneic BMT with ABO, Rh or other red cell antigen mismatches between donor and recipient.