Abstract: Acute graft versus host disease (GvHD) is one of the most significant clinical problems in allogeneic blood and marrow transplantation. Currently, there is no unequivocal diagnostic test for GvHD until the disease is well developed and can be recognized histologically. The invention provides a blood based test for diagnosis and/or prognosis of GvHD, allowing assessment of risk for developing GvHD prior to appearance of clinical symptoms. Using flow cytometry, peripheral blood mononuclear cells are assessed for an increase in proportion and fluctuation of CD3+CD4+CD8?+ cells. An increase in presence or proportion, or high fluctuation in this cell population prior to onset of clinically recognized indicators of GvHD is predictive of later development of GvHD.