Allogeneic bone marrow transplantation is widely used for the treatment of leukemias. Relapse is one of the main complications. Various types of conditioning have been used. Most teams use a combination of cyclophosphamide and different modalities of total body irradiation. In some regimens, high dose alkylating drugs like Busulfan are substituted for radiation. None of these regimens has modified significantly the long term disease free survival, indicating the need for prospective randomized trials.