This is an update of a randomized study on antithymocyte globulin (ATG; Thymoglobulin) before transplantation in patients undergoing unmanipulated marrow transplantation from unrelated donors. The median follow-up for surviving patients is 5.7 years. At last follow-up, chronic graft-versus-host disease (GVHD) was scored in 60% of non-ATG and in 37% of ATG patients (P = .05), and extensive chronic GVHD was present in 41% and 15%, respectively (P = .01). Chronic lung dysfunction was diagnosed in 51% versus 19% of patients (P = .005). Forced vital capacity decreased significantly with time in non-ATG patients (P = .005), but not in patients who received ATG (P = .30). The proportion of patients with Karnofsky scores of ≥90% at 4 years was 57% versus 89% in non-ATG versus ATG patients (P = .03). The actuarial 6-year survival for all patients randomized was 31% versus 44% (non-ATG versus ATG; P = .80). The cumulative incidence of transplant-related mortality was 51% versus 41% (P = .70) and of relapse was 32% versus 40% (P = .90). For patients who survived 1 year, transplant-related mortality was 25% versus 3% (P = .03), and actuarial survival was 58% versus 85% (P = .09). In conclusion, the addition of ATG to cyclosporine/methotrexate provides significant protection against extensive chronic GVHD and chronic lung dysfunction, reduces late transplant mortality, and improves quality of life in patients undergoing unrelated donor transplantation.