Tacrolimus has proved effective for preventing acute graft-vs.-host disease (GVHD) after unrelated donor marrow transplantation, but the therapeutic window is apparently narrow. Therapeutic drug monitoring could potentially be used to guide dose modifications, but the optimal target range of tacrolimus blood concentrations is unknown. We determined whether acute GVHD and renal dysfunction correlated with tacrolimus whole blood levels as measured by the IMx assay. Data were analyzed for 97 adults treated in a multicenter trial of tacrolimus and methotrexate or methylprednisolone as GVHD prophylaxis after unrelated donor marrow transplantation. The rate of grades II-IV GVHD was 49%; 81% of patients had a doubling of the serum creatinine; and 61% had a serum creatinine <2 mg/dL. The initial tacrolimus target range for the clinical trial was 10-60 ng/mL. Tacrolimus blood levels were averaged over a 14-day period, and Cox models were used with averaged blood levels as a time-dependent covariate. No significant change was observed in the risks of acute GVHD, doubling of creatinine, need for dialysis, or death over a tacrolimus blood level range of 5-40 ng/mL, but there was a direct correlation between risk of developing a creatinine <2 mg/dL and increasing tacrolimus blood levels (4.7% increased risk for each 1 ng/mL increase in blood concentration, p < 0.001). When the tacrolimus level exceeded 20 ng/mL, there was a 2.2-fold increase in the rate of renal toxicity (p < 0.001), a trend for an increase in mortality (relative risk 3.9, p=0.08), and no impact on the risk of GVHD. This analysis supports 10-20 ng/mL as the therapeutic range of tacrolimus whole blood steady state or trough levels for unrelated donor marrow transplantation.