To test this they then divided patients according to the median Treg frequency. Patients with Treg frequency below the median had significantly higher nonrelapse mortality (largely attributable to GVHD) compared to patients with Treg frequency above the median 41% vs. 8%, P 5 0.03. This translated into inferior 2 year survival of 38% versus 63% (P 5 0.03). There was no difference in relapse mortality according to Treg frequency. This analysis shows that the frequency of CD4 1 CD25 hi FOXP3 1 Tregs at the onset of GVHD correlates with ultimate severity, nonrelapse mortality, and overall survival. It will be of interest to determine whether Tregs can predict the onset of GVHD and whether therapy can be targeted depending on the frequency of regulatory T cells identified at GVHD onset. This relatively simple and rapid flow cytometry assay may have important diagnostic and prognostic value in patients developing clinical findings consistent with graftversus-host disease after hematopoietic SCT. Abstract Summary GVHD affects 20-70% of allogeneic SCT recipients and can result in significant transplant-related morbidity and mortality due to tissue injury and opportunistic infections from intensive and chronic immunosuppressive therapy [5] [6] [7] . GVHD is initiated by both host and donor antigen presenting cells [including dendritic cells (DC)] and mediated by memory and effector T cells of donor origin. A better understanding of the pathogenesis of GVHD is needed and could lead to more effective preventative or therapeutic interventions and better outcomes from SCT.