Considerable data in animals suggest an anti-leukemia reaction associated with transplantation of allogeneic bone marrow cells. In some instances, this graft-versus leukemia (GVL) reaction may be distinct from graft-versus-host-disease (GVHD). Data in humans also support the concept of a GVL effect associated with bone marrow transplantation. These observations include an increased risk of leukemia relapse in identical twin transplants and in recipients of T cell-depleted allogeneic transplants, and a decreased risk-of leukemia relapse in individuals who develop acute or chronic GVHD. These findings are among the most convincing evidence that the immune system plays a role against cancer in man.