Serum levels of an interleukin-1 receptor family member called suppressor of tumorigenicity 2 (ST2) predict response to therapy for graft-versus-host disease (GVHD) and improve on clinical grading in assessing the risk of death without relapse after allogeneic transplantation. Although mortality related to graft-versus-host disease (GVHD) after allogeneic hematopoietic stem-cell transplantation has been reduced, 1 , 2 acute GVHD remains a major complication of allogeneic transplantation, occurring in approximately half the transplant recipients. 3 , 4 High-dose systemic glucocorticoids remain the first-line therapy for GVHD, 5 – 9 although just half of patients have complete resolution of GVHD by day 28 after therapy initiation. 6 Patients who do not have a response to GVHD therapy are at high risk for death without relapse of the primary disease for which the transplantation was performed within 6 months after therapy initiation. 10 – 13 We previously reported that a model . . .