There is no reliable laboratory indicator of the onset of chronic graft-versus-host disease (cGVHD). This study looks at whether the expression of OX40, a member of the tumor necrosis factor receptor family, is related to the development of cGVHD in patients who underwent allogeneic hematopoietic stem cell transplantation. Peripheral blood mononuclear cells from 22 patients after day 100 were subjected to multicolor flow cytometry. The percentages of both OX40⁺CD4⁺ and OX40⁺CD8⁺T cells were significantly higher in patients with cGVHD than those without (P < .0001 and P = .001, respectively). Serial analyses showed that OX40⁺CD4⁺ T cells elevated before the onset of cGVHD and closely correlated with the therapeutic response. The expression of CD25, CD69, and HLA-DR was partially detectable on OX40⁺ T cells. These results indicate that serial measurement of OX40⁺ T cells is useful for predicting the onset as well as the therapeutic response of cGVHD and raise a possibility that the OX40/gp34 system is involved in the pathogenesis of cGVHD.