The significance and potency of GVL can no longer be argued. It is very clear that an allogeneic bone marrow graft provides an important GVL component critical to the success of BMT for many patients. The extraordinary success of donor MNC infusions to treat relapse after BMT shows that it is now possible to manipulate the GVL reaction to treat leukemia. The identity of the effector cells and target antigens remains unclear, but no doubt future experiments will begin to dissect out the complex cellular and cytokine interactions that mediate GVL reactivity. It also remains unclear whether GVL is distinct from GVHD; ultimately, the ability to harness GVL without excessive toxicity from GVHD will be a central challenge in BMT and cellular immunotherapy. There is now an excellent opportunity to understand the detailed mechanisms of GVL and to begin to design clinical strategies to harness the potent GVL effects of allogeneic donor cells for greater therapeutic benefit.