In our institute, tacrolimus was started at a dose of 0.03 mg/kg/day and adjusted to maintain a blood concentration between 10 and 20 ng/mL combined with short term methotrexate after bone marrow transplantation from an unrelated donor. Dose adjustment was performed strictly, in order to prevent grade II-IV acute graft-versus-host disease (GVHD)while avoiding renal toxicity of tacrolimus. Then, in this study, we retrospectively evaluated the tacrolimus blood concentration during the first 4 weeks after transplantation. The mean tacrolimus concentration of the eligible 52 patients was 17.41+/-4.84(range, 9.5-33.4)ng/mL in the 1st week after transplantation, but declined to 13.7+/- 4.0(range, 8.1-25.6)ng/mL in the 2nd week. The dose of tacrolimus was decreased as follows: 0.022+/-0.005 mg/ kg/day(range 0.011-0.039)in the 1st week, and 0.018+/-0.007 mg/kg/day(range 0.004-0.040)in the 2nd week. The incidence of grade II-IV GVHD was 63.0% and grade III-IV was 13.9%. The individual variations of tacrolimus blood concentration did not affect the incidence of grade II-IV acute GVHD, as far as the concentration being maintained in the range of 14.82+/-4.22 ng/mL during the first 4 weeks after transplantation. In addition, the variations of tacrolimus concentration didn?t associate statistically with renal toxicity.