The most commonly used grading system for acute graft-versus-host disease (aGVHD) was introduced 30 years ago by Glucksberg; a revised system was developed by the International Bone Marrow Transplant Registry (IBMTR) in 1997. To prospectively compare the 2 classifications and to evaluate the effect of duration and severity of aGVHD on survival, we conducted a multicenter study of 607 patients receiving T-cell-replete allografts, scored weekly for aGVHD in 18 transplantation centers. Sixty-nine percent of donors were HLA-identical siblings and 28% were unrelated donors. The conditioning regimen included total body irradiation in 442 (73%) patients. The 2 classifications performed similarly in explaining variability in survival by aGVHD grade, although the Glucksberg classification predicted early survival better. There was less physician bias or error in assigning grades with the IBMTR scoring system. With either system, only the maximum observed grade had prognostic significance for survival; neither time of onset nor progression from an initially lower grade of aGVHD was associated with survival once maximum grade was considered. Regardless of scoring system, aGVHD severity accounted for only a small percentage of observed variation in survival. Validity of these results in populations receiving peripheral blood transplants or nonmyeloablative conditioning regimens remains to be tested.