A graft versus leukaemia (GvL) effect makes a significant contribution to the lower risk of relapse seen in patients after BMT compared with patients receiving chemotherapy alone. Both T cell-dependent and T cell-independent effectors of GvL exist, and both may play an important role in the elimination of minimal residual disease after BMT. There is evidence that GvL activity may be separable from GvHD either by identifying T cell clones recognizing specific leukaemia antigens or by using immunomodulatory drugs or cytokines to enhance T cell-independent GvL mechanisms which operate without alloreactivity and therefore without concomitant exacerbation of GvHD. These approaches should improve survival after both autologous and allogeneic BMT.