The ability of allogeneic bone-marrow cells and peripheral-blood stem cells to cure leukaemia remains the most striking example of the ability of the human immune system to recognize and destroy tumours. However, harnessing this 'graft-versus-leukaemia' effect to improve outcome for patients with advanced disease and segregating it from graft-versus-host disease have proven to be key challenges. The recent identification of molecules that are specifically expressed by leukaemic cells and that can be recognized by T cells has indicated that immunological reactivity can be targeted. This anticancer specificity of T cells should soon be routinely incorporated into allogeneic stem-cell transplant regimens to promote tumour eradication.