Patients with GVHD (N 5 60) had a Treg frequency that was 40% less (0.66% ±0.07, P < 0.001) than those without GVHD and Treg frequency was identified as a significant independent biomarker of GVHD. Furthermore, Treg frequency correlated with the maximum overall grade of GVHD (r 5 20.33; P < 0.001), suggesting Treg values could predict severity and potential prognosis of GVHD.