The aims of this work were both to validate a sensitive and specific method to quantify tacrolimus (TAC) in liver biopsies after hepatic transplantation and to evaluate the predictive value of either tissue or blood TAC concentrations for rejection in 146 adult patients under a TAC-based immunosuppression. Trough blood levels were monitored daily during the hospital stay by immunoassay. Liver biopsies were routinely performed at day 7 posttransplantation. The tissue assay was developed by liquid chromatography-mass spectrometry. The limit of quantification was 5 pg/mg, with intra- and interassay precision ranging from 3.9% to 14.3% and 4.7% to 15.9%, respectively. The extraction efficiency was approximately 80%. TAC found in liver biopsies ranged from less than 5 up to 387 pg/mg. Blood TAC levels ranged from 2.7 to 19.3 ng/mL. Tissue levels displayed excellent correlation with liver histopathologic BANFF rejection score, whereas blood levels did not. Clinically significant rejections (BANFF scores > or = 6) were characterized by mean TAC tissue and blood concentration of 13.1 pg/mg and 7.6 ng/mL, respectively, whereas these mean values became, respectively, 74.9 pg/mg (P < 0.05) and 7.1 ng/mL (not significant) for nonclinically significant rejection episodes (BANFF < 6). In this study, hepatic tissue TAC concentrations were distributed in a wider range and displayed a significantly better correlation with the severity of the organ rejection than predose blood levels. A tissue TAC concentration less than 30 pg/mg is 89% sensitive and 98% specific to discriminate clinically significant cellular rejection. Further studies are required to better understand the factors affecting TAC distribution within liver tissue (such as carrier proteins and cytochrome genetic polymorphism, liver function, age, hepatic blood flow, type of organ transplanted, time posttransplantation) and to define its value in the treatment of liver allograft rejection.