Infliximab is a chimeric anti-tumor necrosis factor-alpha monoclonal antibody. Infusion related reactions and infection are well known side effects of infliximab; however, renal complications have not been well recognized. We report on a patient with late onset-acute tubulointerstitial nephritis (ATIN) after treatment with infliximab and mesalazine for Crohn's disease. A 25-year-old woman was admitted with a purpuric rash on both lower extremities and arthralgia. She had been diagnosed with Crohn's disease 5.6 years previously and had been treated with mesalazine and infliximab. Serum creatinine level, last measured one year ago, was elevated from 0.6 mg/dL to 1.9 mg/dL. Results of urinalysis, ultrasound, and serologic examinations were normal. With a tentative diagnosis of Henoch-Schonlein purpura, oral prednisolone was given, and serum creatinine decreased to 1.46 mg/dL, but was elevated to 2.6 mg/dL again at two months after discontinuation of prednisolone. Renal biopsy indicated that ATIN was probably induced by drug, considering significant infiltration of eosinophils. Concomitant use of infliximab with mesalazine was supposed to trigger ATIN. Oral prednisolone was administered, and serum creatinine level showed partial recovery. Thus, ATIN should be suspected as a cause of renal impairment in Crohn's disease even after a long period of maintenance treatment with infliximab and mesalazine.