Actuarial probability of acute GVHD grade III-IV in trial 1 (upper graph) and in trial 2 (lower graph).

Trial 1 compared 7.5 mg/kg antithymocyte globulin (ATG) (given over 2 days) to patients not receiving ATG. Trial 2 compared 15 mg/kg ATG (given over 4 days) to patients not receiving ATG. There was no difference in the risk for GVHD III-IV in trial 1 but a significant difference in trial 2.