Newly diagnosed patients with acute graft-versus-host disease (GvHD, grades I-IV; n = 211) were given 6-methylprednisolone (6MPred) 2 mg/kg per day for 5 consecutive days; 150 patients (71%) tapered 6MPred on day +5 and were considered responders; 61 patients (29%) could not taper their steroid dose and were considered nonresponders. The cumulative incidence of transplant-related mortality (TRM) for responders and nonresponders is, respectively, 27% and 49% (P = .009), and the 5-year survival is 53% and 35% (P = .007). Nonresponders on day +5 (n = 61) were randomized to receive 6MPred 5 mg/kg per day for 10 days alone (n = 34) or in combination with rabbit anti-thymocyte globulin (ATG, 6.25 mg/kg in 10 days; n = 27). The 2 groups were balanced for clinical and GvHD characteristics. One month after randomization, 26% had a complete response; 23%, a partial response; 33%, stable GvHD; 10%, worsened; and 8%, died. There was no significant difference in response, TRM, and survival between the non-ATG and ATG group. In conclusion, 5 days of prednisolone as first-line therapy of acute GvHD identifies patients with different risk of TRM, and second-line therapy with a combination of 6MPred + ATG does not improve patient outcome, compared with 6MPred alone.