We report a case of encephalopathy seemingly caused by tacrolimus (FK506) in spite of blood concentration near the upper limit of therapeutic range. A 26-year-old man received FK506 to prevent acute graft-versus-host disease after hematopoietic stem cell transplantation. He underwent an intravenous injection of FK506 the day before transplantation (day -1). He developed headache, hypertension, nausea and vomiting from day 2 to day 3. A computed tomography scan showed a low density area with unclear border in the bilateral cerebellar hemispheres. Thereafter, these symptoms improved with discontinuation of FK506, which was strongly suggestive of encephalopathy caused by FK506. The blood concentration of FK506 at the onset of encephalopathy was 21.7 ng/mL. Although this value was slightly higher than the standard therapeutic range (10-20 ng/mL), it was within clinically acceptable range in the early stage after stem cell transplantation. This indicates that even if the blood concentration of FK506 is within the therapeutic range, encephalopathy may develop. In summary, although the blood concentration of FK506 is useful as an indicator for prevention of encephalopathy, we propose careful monitoring not only of the blood concentration but also clinical status for the detection of initial symptoms and prevention of aggravation.