Granulysin is a newly identified CTL/NK cell-related cytotoxic protein, which is secreted in both constitutive and Ca-dependent manner. To evaluate its significance in stem-cell transplantation (SCT), serum granulysin was measured by newly established ELISA method in 26 patients undergoing SCT (21 allogeneic and 5 autologous). In the allogeneic SCT, granulysin was transiently increased in 3 weeks, which was not observed in autologous SCT. In acute GVHD, serum granulysin was markedly increased and correlated with the severity of GVHD. Elevation of granulysin was not necessarily associated with increase of sIL2R or IFN-γ. In vitro, allospecific T cells released granulysin in an allo-specific manner, and it was correlated with allo-specific cytotoxic activity. These results indicate that increased release of granulysin presents alloreactivity and serum granulysin is useful as a marker of GVHD in SCT. Am. J. Hematol. 81:340-348, 2006. © 2006 Wiley-Liss, Inc.