Many patients suffer febrile diseases soon after allogeneic stem cell transplantation (SCT). Some of the symptoms of viral infections and acute GVHD are often difficult to distinguish. However, an accurate diagnosis is important since the treatments for these conditions are different. It is known that MxA protein is specifically induced in patients with several viral infections. We investigated the cytoplasmic expression of MxA in the peripheral blood mononuclear cells (PBMCs) of patients with fever after allogeneic SCT using a newly generated monoclonal antibody (KM1135) and flow cytometry. The level of MxA expression was significantly higher in patients diagnosed with viral infections (n=6, cytomegalovirus in three, Epstein-Barr virus in one, human herpesvirus-6 in one, adenovirus in one) than control individuals (n=9) (P<0.05, Mann-Whitney test). The level of MxA in patients with aGVHD (n=7) was identical to that in controls. The level of MxA correlated well with the amount of the cytomegalovirus antigen-positive cells in the presence of acute GVHD in two patients. The measurement of MxA is simple and useful in distinguishing viral disease from acute GVHD after allogeneic SCT.