For severe aplastic anemia and several malignant hemopathies allogeneic bone marrow transplantation is the only treatment with curative potential. This is the case for chronic myelogenous leukemia, the myelodysplastic syndromes and probably multiple myeloma and chronic lymphocytic leukemia. It seems also the best therapeutic option for young adults who suffer from acute leukemia and for whom an adequate family donor is available. We review here the main complications of the procedure. Their better knowledge and the way to prevent and to treat them has decreased the mortality and morbidity of this treatment which is mostly successful when applied on patients in the early phase of their disease. Recently, the availability of HLA typed registered volunteers has extended the applicability of allogeneic bone marrow transplantation for those patients who lack adequate familial donors.