Immune mechanisms superimposed to the myeloablative conditioning regimens exert an additional powerful effect in eradicating leukemia and in achieving immunological control of minimal residual disease. The impact of GVHD-independent GVL has been evaluated to be absent, or near absent, in ALL, about 30% in AML and about 40% in CML. While until little time ago most of the evidence in favor of an immune antileukemia mechanism exerted by allo BMT in CML was indirect, based on the lack of GVL, there is now solid evidence of a positive type, based on the antileukemia effect of donor lymphocyte infusions in patients having relapsed after transplant. There are three lines of indirect clinical evidence for GVL in CML: they include the classical linkage between GVHD and reduced relapse rate, increased relapse rate after identical twin allografts, and increased relapse risk after effective GVHD prophylaxis, with T lymphocyte depletion in the foreground. The eradicating effects of donor lymphocyte infusions in relapsed patients are the ultimate demonstration that allogeneic immune competent cells are capable of recognizing and destroying the Ph-positive clone. However the frequency of irreversible aplasia indicates that donor lymphocytes act in the same way on residual host hematopoiesis, so that a second graft, without repeat conditioning, should be programmed for such cases.