This report investigated the impact of graft-versus-host disease (GVHD) on transplantation outcomes in 1859 acute myeloid leukemia patients given allogeneic peripheral blood stem cells after reduced-intensity conditioning (RIC allo-SCT). Grade I acute GVHD was associated with a lower risk of relapse (hazards ratio (HR) ¼ 0.7, P ¼ 0.02) translating into a trend for better overall survival (OS; HR ¼ 1.3; P ¼ 0.07). Grade II acute GVHD had no net impact on OS, while grade III-IV acute GVHD was associated with a worse OS (HR ¼ 0.4, Po0.0.001) owing to high risk of nonrelapse mortality (NRM; HR ¼ 5.2, Po0.0001). In time-dependent multivariate Cox analyses, limited chronic GVHD tended to be associated with a lower risk of relapse (HR ¼ 0.72; P ¼ 0.07) translating into a better OS (HR ¼ 1.8; Po0.001), while extensive chronic GVHD was associated with a lower risk of relapse (HR ¼ 0.65; P ¼ 0.02) but also with higher NRM (HR ¼ 3.5; Po0.001) and thus had no net impact on OS. In-vivo T-cell depletion with antithymocyte globulin (ATG) or alemtuzumab was successful at preventing extensive chronic GVHD (Po0.001), but without improving OS for ATG and even with worsening OS for alemtuzumab (HR ¼ 0.65; P ¼ 0.001). These results highlight the role of the immune-mediated graft-versus-leukemia effect in the RIC allo-SCT setting, but also the need for improving the prevention and treatment of severe GVHD.