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Although haematopoietic stem cell transplantation ( HSCT) is a potential curative treatment for haematological malignancies, it is still a procedure associated with substantial morbidity and mortality due to toxicity, graft-versus-host disease ( GVHD) and relapse. Recent attempts of developing safer transplantation modalities increasingly focuses on selective cell depletion and graft engineering with the aim of retaining beneficial immune donor cells for the graft-versus-leukaemia ( GVL) effect. In this context, the adoptive and especially innate effector functions of γδ T cells together with clinical studies investigating the effect of γδ T cells in relation to HSCT are reviewed. In addition to phospho-antigen recognition by the γδ T cell receptor ( TCR), γδ T cells express receptors of the natural killer ( NK) and natural cytotoxicity ( NCR) families enabling them to recognize and kill leukaemia cells. Antigen recognition independent from the major histocompatibility complex ( MHC) allows for the theoretical possibility of mediating GVL without an allogeneic response in terms of GVHD. Early studies on the impact of γδ T cells in HSCT have reported conflicting results. Recent studies, however, do suggest an overall favourable effect of high γδ T cell immune reconstitution after HSCT; patients with elevated numbers of γδ T cells had a significantly higher overall survival rate and a decreased rate of acute GVHD compared to patients with low or normal γδ T cell counts. Further research in terms of effector mechanisms, subtypes and tissue distribution during the course of HSCT is needed to assess the potentially beneficial effects of γδ T cells in this setting. [ABSTRACT FROM AUTHOR]