Disclosed is a method of diagnosing graft-versus-host disease, comprising measuring the level of CCL8 protein in a sample obtained from a subject as an indicator for the diagnosis or course of graft-versus-host disease. Also a diagnostic reagent for graft-versus-host disease comprising an anti-CCL8 antibody is disclosed. The method of the present invention enables diagnosis of the onset of graft-versus-host disease and monitoring of the progress, in particular, differential diagnosis between graft-versus-host disease and an infectious disease. The present invention also provides a method for treating graft-versus-host disease utilizing the anti-CCL8 antibody.
