Intracellular tacrolimus (Tac) concentrations may better correlate with its biological effect than whole blood concentrations. Therefore the monitoring of intracellular concentrations of Tac offers a novel and promising approach to individualize treatment after organ transplantation. Our purpose is to investigate the relationship between Tac peripheral blood mononuclear cell (PBMC) drug concentrations and whole blood concentrations of Tac, and the impact of genetic polymorphisms in a metabolizing enzyme (CYP3A5) and a transporter protein (ABCB1) on Tac PBMC drug concentrations. Also, whole blood Tac concentration and PBMC concentrations will be compared with respect to their correlation with transplant outcomes.