Acute graft-versus-host disease (aGVHD) after allogeneic bone marrow transplantation (BMT) is mediated by grafted T lymphocytes after their polarization into type 1 T cells. Interleukin (IL)-18, a novel immunoregulatory cytokine, strongly stimulates type 1 T cells, therefore we postulated that IL-18 may be involved in the pathogenesis of aGVHD. Using an enzyme-linked immunosorbent assay (ELISA), we serially measured serum levels of IL-18 in 37 patients with haematological malignancy before and after allogeneic BMT. Patients with aGVHD had high levels of IL-18 that strongly correlated with the severity of aGVHD. We also found that they showed reduced serum levels of IL-18 after appropriate treatment or at a state of resolution. IL-18 levels were not affected by the pretransplant regimen, engraftment or bacterial infection. Compared with circulating interferon (IFN)-gamma or IL-12 levels, serum levels of IL-18 showed a more sensitive and specific correlation with the disease status of aGVHD. These findings suggest that IL-18 may play important roles in the pathogenesis of aGVHD and that measurement of serum IL-18 levels can be useful indicator of aGVHD.