The remarkable activity of donor T cells against malignant cells in the context of an allogeneic haematopoietic stem-cell transplantation (HSCT) is arguably, at present, the most potent clinical immunotherapy for cancer. However, alloreactive donor T cells are also important effector cells in the development of graft-versus-host disease (GVHD), which is a potentially lethal complication for recipients of an allogeneic HSCT. Therefore, the separation of the GVHD and graft-versus-tumour (GVT) activity of donor T cells has become a topic of great interest for many investigators. Recent studies have shown that donor T cells make differential use of their cytolytic pathways in mediating GVHD and GVT effects. Therefore, the selective blockade or enhancement of cytolytic pathways provides an intriguing therapeutic opportunity to separate the desired GVT effect from the potentially devastating GVHD.