We studied the impact of preparative regimens with or without antithymocyte globulin (ATG) on chronic GVHD in 160 patients undergoing marrow transplants from unrelated donors (n = 127) or partially mismatched related donors (n = 33). A conditioning regimen that included rabbit ATG, 7.5 to 15 mg/kg (Thymoglobuline; Sangstat, Lyon, France), was given to 102 patients, whereas a conditioning regimen without ATG was given to 58 patients. The median patient age was 34 years for the ATG group and 29 years for the non-ATG group (P = .002); otherwise the 2 groups were matched for disease phase, diagnosis, donor age, interval from diagnosis to transplantation, and number of cells infused at the time of transplant. Median follow-up for surviving patients was 4.5 years (range, l.5-9 years). The conditioning regimen was cyclophosphamide (CY) and total body irradiation (TBI) in 95 patients and CY-thiotepa in 65 patients; the source of stem cells was bone marrow for all patients. Acute GVHD grades II-IV and grades III-IV were reduced in patients receiving ATG compared to patients not receiving ATG (51% versus 74%, P = .004 and 14% versus 28%, P = .03, respectively). There were significantly fewer patients with chronic GVHD in the ATG group than in the non-ATG group at 6 months (14% versus 30%, P = .03), 1 year (7% versus 41%, P = .0001), 2 years (16% versus 36%, P = .02), and 4 years (5% versus 34%, P = .002) and beyond 4 years (0% in 19 patients at risk versus 29% in 24 patients at risk, P = .01). More patients in the ATG group than in the non-ATG group had a performance status (Karnowski score) greater than 90 at last follow-up (93% versus 56%, P = .01) and had discontinued cyclosporin treatment 2 years posttransplant (28% versus 3%, P = .003). Survival rates were comparable in the ATG and non-ATG groups for patients who received TBI (56% versus 59%, P = .7) and those who received thiotepa (33% versus 18%, P = .3). Transplant mortality and relapse rates were also comparable in the 2 groups for these patients. We conclude that pretransplant ATG administration reduces the risk of acute and chronic GVHD, improves quality of life, and increases the likelihood that discontinuation of immunosuppressive therapy will be possible.