Second-line therapies for steroid-resistant acute GVHD have been used with limited success. We have reviewed the responses of 79 hematopoetic stem cell transplant (HSCT) patients uniformly treated from 1990-1998 with equine antithymocyte globulin (ATG) for steroid-resistant acute GVHD, defined as progression of acute GVHD after 4 days of treatment with prednisone or no improvement of acute GVHD after 7 days of treatment with prednisone. Patients received HSCT from 34 related (32 matched sibling/2 partially matched) and 45 unrelated (14 HLA-A, -B, -DRB1 matched/31 partially matched) donors. Prior to ATG therapy, severe (grade III-IV) GVHD was observed in 34 patients (43%). Organs involved included skin in 81% of patients, lower GI tract in 52%, upper GI tract in 28%, and liver in 11%. Treatment consisted of 1-5 courses (median, 2 courses) of ATG (15 mg/kg per dose bid x 5 days) given for a median of 16 days (range, 5 to 44 days) after the onset of GVHD. All patients continued to receive prednisone, 60 mg/m2 per day (or methylprednisolone IV equivalent), plus CSA (75%) or tacrolimus (4%). At day 28 of treatment, overall improvement was observed in 54% of patients; durable (> or = 28 days) complete response was observed in 20% of patients, and partial response was observed in 34% of patients. In multivariate analysis, patients with CML or a malignant disease other than acute leukemia had a greater likelihood of overall response than did those with nonmalignant diseases. Patients with acute skin GVHD (with or without other organ involvement) responded most frequently. Chronic GVHD developed in 51% of patients by 1 year after HSCT. One patient developed EBV lymphoproliferative disease. For the entire cohort, the probability of survival at 1 year was 32% (95% CI, 22%-42%). In multivariate analysis, factors associated with better survival included earlier onset of acute GVHD, shorter time from initial treatment for GVHD to treatment with ATG, and the use of non-T-cell-depleted stem cell grafts. These data suggest that treatment with ATG can be an active therapy, especially in patients with skin GVHD and early signs of steroid resistance.