This article reports changes in tacrolimus (FK506) blood levels connected with carbamazepine (CBZ). A drug interaction between FK506 and CBZ was investigated in a woman, who was in her 40s, who underwent heart transplantation. Pharmacokinetic parameters were measured, including dose and trough blood levels (C(0)), area under the serum concentration-time curve from 0 to 12 hours (AUC(0-12h)), and apparent clearance of oral FK506 (CL/F) for FK506 alone (about 3 months before starting CBZ) and combined with CBZ (11 days and about 3 months after starting CBZ). FK506 C(0) levels were decreased within 7 days of CBZ treatment. FK506 dosing required a 1.3- to 1.4-fold increase to maintain adequate blood levels while taking 200 mg CBZ daily. The AUC(0-12h)/dose 11 days after CBZ treatment was about 50% of the value before CBZ, and was about 70% at 3 months after CBZ treatment. The CL/F at 11 days and about 3 months after starting CBZ treatment was about 2 times higher than before CBZ therapy. FK506 C(0) levels are decreased by CBZ treatment, and blood levels should be closely monitored.