A 51-year-old female received a kidney transplant, donated by her husband. The patient was induced with tacrolimus, mycophenolate mofetil and prednisolone. After methyl prednisolone pulse therapy without biopsy, allograft biopsy on POD 160 showed severe tubulo-interstitial nephritis with intranuclear inclusions. Urine cytology also showed decoy cells. Blood PCR detected an increase of BK virus DNA. She was diagnosed as having BK virus-associated nephropathy . Reduction of tacrolimus and switching of mycophenolate mofetil to mizoribine were done. Serum Creatinin (sCr) still rose to 3.0 mg/dl with persistent viremia and viruria. From on POD 268, 0.25 mg/kg of cidofovir was administered intravenously every two weeks over about four months. Biopsy on POD 387 revealed the disappearance of tubulitis with intranuclear inclusions, and decoy cells also disappeared from urine cytology. BK virus DNA in the blood decreased under the threshold level. sCr was stable and remained about 2.2 mg/dl for three months after the final treatment of cidofovir.