The combination of two powerful immunosuppressive agents, methotrexate (MTX) and cyclosporine (CSP), has resulted in a significant decrease in the morbidity and mortality after allogeneic bone marrow transplantation (BMT). However, the additive toxicities from ablative preparative regimens may lead to suboptimal use of this combined immunoprophylaxis. We evaluated the efficacy and feasibility of administering MTX/CSP with busulfan (4 mg/kg/d for 4 days) and cyclophosphamide (50 mg/kg/d for 4 days) (BuCy4) in 101 consecutive patients with hematologic malignancies categorized into high- and low- risk groups receiving HLA-matched marrow grafts. Postgrafting immunosuppression consisted of MTX short course (15 mg/m2 on day 1 and 10 mg/m2 on days 3, 6, and 11) and intravenous CSP (1.5 mg/kg every 12 hours). Eighty-three patients (82.1%) received 100% of MTX calculated dose and 87 (86.1%) achieved a CSP therapeutic level (250 to 600 ng/mL) within a median of 16 days. Seventy-three patients (72.2%) received optimal immunosuppressive therapy comprising a full MTX course and achieving CSP therapeutic concentrations. The Kaplan-Meier estimated incidence of grade II to IV acute graft-versus-host disease (GVHD) was 9.2% for all patients and 5.5% in patients receiving optimal GVHD prophylaxis. Eighty-nine patients (88.2%) survived > or = 100 days posttransplant and 43 (48.3%) developed chronic GVHD, the majority of which were de novo (31 of 43). The estimated incidence of relapse was 28.9% for all patients and 14.8% in the low-risk group, with a median follow-up of 24.5 months. High-risk features and the absence of chronic GVHD were significantly associated with relapse (P = .002 and .036, respectively) in multivariate analyses. Projected disease-free survival at 2 years was 52.3% for all patients and 65.2% in low-risk patients. Disease-free survival was significantly improved in optimally treated patients (P = .03) due to a lower incidence of early deaths from acute GVHD and infectious episodes. In conclusion, optimal delivery of MTX/CSP in association with BuCy4 resulted in a near complete abrogation of acute GVHD in HLA-matched transplants and a significantly improved disease-free survival.