Hepatocyte growth factor (HGF) was reported to be effective in preventing acute graft-versus-host disease (GVHD) in a murine model. We examined serum HGF concentrations in 38 patients receiving allogeneic bone marrow transplants, and investigated the relationship of serum HGF concentrations to severity of acute GVHD. More HGF was present in sera from patients with than without acute GVHD. Serum HGF correlated significantly with grade of acute GVHD. Furthermore, serum HGF correlated with serum concentrations of C-reactive protein, gamma-glutamyltranspeptidase (GTP), and aspartate aminotransferase (AST). Serum concentrations of HGF in transplanted patients without GVHD were consistently low, while those in patients with acute GVHD increased with exacerbation. We conclude that HGF was produced during induction of the GVH reaction, and probably increased as a physiological response.