ICOS, a CD28 family member expressed on activated CD4+ and CD8+ T cells, plays important roles in T cell activation and effector function. Here we studied the role of ICOS in graft-vs-host disease (GVHD) mediated by CD4+ or CD8+ T cells in allogeneic bone marrow transplantation. In comparison of wild-type and ICOS-deficient T cells, we found that recipients of ICOS−/− CD4+ T cells exhibited significantly less GVHD morbidity and delayed mortality. ICOS−/− CD4+ T cells had no defect in expansion, but expressed significantly less Fas ligand and produced significantly lower levels of IFN-γ and TNF-α. Thus, ICOS−/− CD4+ T cells were impaired in effector functions that lead to GVHD. In contrast, recipients of ICOS−/− CD8+ T cells exhibited significantly enhanced GVHD morbidity and accelerated mortality. In the absence of ICOS signaling, either using ICOS-deficient donors or ICOS ligand-deficient recipients, the levels of expansion and Tc1 cytokine production of CD8+ T cells were significantly increased. The level of expansion was inversely correlated with the level of apoptosis, suggesting that increased ability of ICOS−/− CD8+ T cells to induce GVHD resulted from the enhanced survival and expansion of those cells. Our findings indicate that ICOS has paradoxical effects on the regulation of alloreactive CD4+ and CD8+ T cells in GVHD.