To evaluate the implications of soluble Fas ligand (sFasL) in acute graft-versus-host disease (aGVHD) and discriminating symptoms of aGVHD from those of infection, plasma levels of sFasL were assessed in 84 plasma samples from 13 patients who had undergone allogeneic BMT by using a sandwich enzyme-linked immunological assay (ELISA). Plasma sFasL levels of the patients before BMT and at different time points during the post-BMT period were measured. Three points were concluded: (1) Plasma sFasL levels were higher in patients with grade II-IV aGVHD than in those with grade 0 or I aGVHD. (2) Plasma sFasL levels in patients with infection were not statistically different from those in patients without infection. (3) In seven patients with grade II-IV aGVHD, the plasma sFasL levels at pre-BMT were much lower than those of the six patients with 0 or I grade aGVHD. sFasL may be useful for the diagnosis of aGVHD and for differentiating aGVHD from other BMT related complications such as infection.