To investigate the potential role of interleukin 12 (IL 12) in human acute graft versus host disease (aGVHD). 26 patients undergoing allogeneic hematopoietic stem cell transplantation (allo HSCT) were included in this study. IL 12 protein and gene expression in peripheral blood mononuclear cells were measured using enzyme linked immunosorbent assay (ELISA)and reverse transcription polymerase chain reaction (RT PCR), respectively. The results showed that all the patients achieved engraftment.Among them, 10 patients developed grade ⅠGVHD, 5 patients deveolped grade Ⅲ or Ⅳ GVHD. IL 12 protein and gene expressions in peripheral blood mononuclear cells were significantly higher in patients with aGVHD compared with those without aGVHD.The levels of IL 12 correlated significantly with the severity of aGVHD .The results suggested that IL 12 might play an important role in the development of human aGVHD.