Infliximab (IFX) is used as a therapeutic agent for ulcerative colitis (UC) and Crohn's disease (CD). Although the dosage regimen has been established through clinical trial experience, it has yet to be assessed with a pharmacokinetic and pharmacodynamic model. The present study analysed sequential changes of clinical response in patients with ulcerative colitis and Crohn's disease following repeated administrations of infliximab using the pharmacokinetic/pharmacodynamic model. In addition, the dosage regimen presently used for patients with ulcerative colitis was evaluated, as well as the potential efficacy gained by increasing the dose and/or reducing the interval of administration for patients with Crohn's disease. Furthermore, the possibility of evaluating the difference between both diseases with regard to the efficacy of infliximab was investigated. Sequential changes in the clinical response values obtained with our model were in good agreement with the observed values following administration of infliximab in patients with ulcerative colitis and Crohn's disease. The results showed the importance of a loading dose for patients with ulcerative colitis, as well as the efficacy of increasing the dose and reducing the interval for patients with Crohn's disease. Also, the efficacy of infliximab for both diseases is suggested to be similar. In conclusion, our results show a possible modeling scenario that can accommodate the clinical response to infliximab administered for ulcerative colitis and Crohn's disease. Furthermore, it provides confirmation for the present dosage regimens given for these diseases.