A combination of tacrolimus (TAC) and sirolimus (SIR) has recently proved to be a very effective immunosuppressive regimen in organ transplantation. In pediatric transplant recipients, co-administration of these two drugs has been shown to result in a significant decrease of exposure to TAC, whereas conflicting data have been obtained regarding this pharmacokinetic interaction in adults. The aim of this study was to investigate the effect of SIR on TAC pharmacokinetics in adult transplant recipients. Sixteen adult patients (mean age 38+/-8 years), who had been on standard TAC plus low-dose SIR immunosuppressive treatment for 6 months after renal transplantation, were enrolled for a TAC pharmacokinetic study before and 15 days after discontinuing SIR. Eight patients had received SIR 0.5 mg day(-1) and eight patients 2 mg day(-1). TAC doses remained the same in all patients after SIR withdrawal. After discontinuing SIR, statistically significant, dose-dependent increases were observed in area under the curve (AUC), peak (C(max)) and trough (C(min)) TAC concentrations (+15-20% and +27-32%, after discontinuing the 0.5 and the 2 mg day(-1) doses, respectively). Proportional decreases were consistently observed in apparent oral clearance (-13% and -23%). Very good correlations were found between TAC AUC and C(min), both before and after SIR withdrawal (R(2)=0.94, P<0.0001 and R(2)=0.97, P<0.0001, respectively). Our findings clearly demonstrate that the SIR-induced reduction in TAC exposure also takes place in adults and is, therefore, a general, age-independent phenomenon. Hence, TAC levels need to be carefully monitored in transplant recipients of any age, in order to avoid possible TAC overexposure upon SIR discontinuation.