Thirty adults with leukemia or lymphoma undergoing marrow transplantation from HLA-compatible unrelated donors received tacrolimus (FK506), a new immunosuppressive macrolide lactone, and minidose methotrexate to prevent acute graft-versus-host disease (GVHD). The group had a median age of 36 years (range 21 to 49 years). Twenty-four patients had advanced disease, and 11 were resistant to conventional therapy. Tacrolimus was administered at 0.03 mg/kg/d intravenously (i.v.) by continuous infusion from day -2, converted to oral at four times the i.v. dose following engraftment, and continued through day 180 posttransplant. Methotrexate 5 mg/m2 was given i.v. on days 1, 3, 6, and 11. All patients engrafted. Grades 2–4 GVHD occurred in 34% (95% CI, 17% to 52%), and grades 3–4 GVHD in 17% (95% CI, 3% to 31%). Mild renal toxicity was common before day 100; 63% of patients had a doubling of creatinine, and 52% had a peak creatinine greater than 2 mg/dL, but only one patient was dialyzed. The median last i.v. dose of tacrolimus was 53% of the scheduled dose, and the median oral dose on day 100 was 41% of that scheduled. Overall survival at 1 year was 47% (95% CI, 27% to 66%). We conclude that tacrolimus can be combined safely with minidose methotrexate, and the combination has substantial activity in preventing acute GVHD after unrelated donor marrow transplantation.