Pharmacokinetic/pharmacodynamic (PK/PD) modeling of antitumor agents has been developed for doxorubicin (DOX) in order to predict the optimum conditions for a drug carrier to maximize the antitumor effect. A PK model was constructed for free and liposomal doxorubicin using a hybrid model wherein the disposition in the whole body is described by compartment models, which were linked to the tumor compartment via the blood flow rate. The PD model for doxorubicin was described by a cell-kill kinetic model, which represents the number of tumor cells quantitatively, as a function of the free concentration of doxorubicin in the tumor compartment. The influence of each parameter on the antitumor effects was examined by sensitivity analysis based on the PK/PD model, which clearly showed the importance of optimizing the release rate of DOX from liposomes. The validity of the model has been tested using animal experiments. Preliminary simulations were also performed for humans after scaling up the PK/PD model from rodents to humans. The optimum conditions in the rate of drug release from liposomes were different for rodents vis-a-vis humans, which indicates the limitations involved in extrapolating optimum conditions for experimental animals to those for humans.