Acute graft-versus-host disease (aGVHD) is one of the major problems following allogeneic peripheral blood stem cell transplantation (allo-PBSCT). In order to investigate the pathogenesis of human aGVHD, we analyzed cytokine gene expression and cytokine secretion by peripheral blood mononuclear cells (PBMC) in 30 patients who underwent allo-PBSCT. In this study, reverse transcription-polymerase chain reaction (RT-PCR) was used to explore the mRNA expression of interleukin (IL)-2, interferon (IFN)-gamma, IL-4, IL-10 and IL-12 in the PBMC of allo-PBSCT patients with aGVHD and in controls. The concentrations of these cytokines and of IL-18 were also measured by means of ELISA in medium obtained from cultured leukocytes after stimulation with PHA or LPS. Compared with the normal allo-PBSCT group (n=14), IL-2 and IFN-gamma were detected more frequently in aGVHD group (n=16). IL-12 and IL-18 were elevated, while IL-10 level decreased in the same group. There is no difference in IL-4 gene expression between patients with or without aGVHD, but the concentration of IL-4 for patients with grade II-IV aGVHD decreased. These data suggest that IL-2, IFN-gamma, IL-12 and IL-18 play important roles in the development of aGVHD in humans.