We have evaluated the use of blood stem cell grafts for rapid hematopoietic recovery and tacrolimus (FK506) as GVHD prophylaxis to reduce early mortality after allogeneic transplantation. Eighty-five adults with advanced leukemia received high-dose thiotepa, busulfan, and cyclophosphamide as a preparative regimen in a prospective Phase II study. All donors were HLA-matched and related. Marrow (BMT) was used for 44 patients and filgrastim-mobilized blood stem cells (SCT) for 41 patients. GVHD prophylaxis consisted of cyclosporine (CsA) or FK506 with methotrexate (MTX) or methylprednisolone (MP). The median time to neutrophil recovery was earlier after SCT than after BMT (day 10 vs. 17, P<0.001), but this was due to the selective use of MTX only in the BMT patients. The risk of grades 2-4 GVHD was lower with FK506 than with CsA (16% vs. 45%, P=0.02) and was the same for SCT recipients as for BMT recipients (33% vs. 34%). Regimen-related toxicity was significantly lower after SCT than after BMT but did not differ between the FK506 and CsA patients. In comparison with those receiving the standard transplant (BMT with CsA and MTX), only the SCT recipients using FK506 and MP had a significantly higher survival at day 180 posttransplant (84% vs. 53%, P=0.014). In multivariate analyses, use of FK506 was associated with a lower risk of treatment-related mortality and a higher survival at day 180, while the diagnosis of acute lymphoblastic leukemia was associated with a higher risk of treatment-related mortality. These data suggest that the use of blood stem cell grafts and FK506 can reduce the early mortality after allogeneic transplantation for advanced leukemia.