Experience with 17 allogeneic bone marrow transplantation (BMT) patients with refractory acute leukemia is reported. The authors show that it is extremely difficult to cure patients with end stage disease by this procedure. Based on experience that, if performed early, BMT can bring about over 50% potential cures, it is postulated that marrow grafts should be performed much earlier than is done at present. From the results in 45 patients with severe aplastic anemia it is concluded that the majority do not have a defect at the pluripotent stem cell itself. There is disturbed maturation to functional end cells which can be corrected in most instances by ALG, infusion of allogeneic marrow and low dose androgens. BMT between HLA-identical siblings need not cause major problems, but it involves a considerable risk of fatal complications such as irreversible marrow rejection and GvH.