Allogeneic haematological stem cell transplantation (HSCT) has developed into immunotherapy. Donor CD4⁺, CD8⁺ and natural killer (NK) cells have been reported to mediate graft‐versus‐leukaemia (GVL) effects, using Fas‐dependent killing and perforin degranulation to eradicate malignant cells. Cytokines, such as interleukin‐2, interferon‐γ and tumour necrosis factor‐α potentiate the GVL effect. Post‐transplant adoptive therapy of cytotoxic T‐cells (CTL) against leukaemia‐specific antigens, minor histocompatibility antigens, or T‐cell receptor genes may constitute successful approaches to induce anti‐tumour effects. Clinically, a significant GVL effect is induced by chronic rather than acute graft‐versus‐host disease (GVHD). An anti‐tumour effect has also been reported for myeloma, lymphoma and solid tumours. Reduced intensity conditioning enables HSCT in older and disabled patients and relies on the graft‐versus‐tumour effect. Donor lymphocyte infusions promote the GVL effect and can be given as escalating doses with response monitored by minimal residual disease. A high CD34⁺ cell dose of peripheral blood stem cells increases GVL. There is a balance between effective immunosuppression, low incidence of GVHD and relapse. For instance, T‐cell depletion of the graft increases the risk of relapse. This paper reviews the current knowledge in graft‐versus‐cancer effects. Future directions, such as immunotherapy using leukaemia‐specific CTLs, allo‐depleted T‐cells and suicide gene manipulated T‐cells, are presented.