Tubulointerstitial nephritis caused by polyomavirus of the subtype BK (BK virus nephropathy, BKN) is an important cause of deterioration of renal allograft function after kidney transplantation. In 3 cases of BKN diagnosed at our center, the suspected diagnosis made on the basis of urine cytology and serum PCR was confirmed by electron microscopy and immunohistology of the renal graft biopsy. In 1 patient, stable renal function without further virus detection was seen after reduction of the immunosuppression. In 2 further patients there was loss of graft function. BKN is an important differential diagnosis of unclear deterioration of renal graft function. The risk is particularly high with use of tacrolimus and mycophenolate mofetil (MMF). Urine cytology and serum PCR are suitable screening tests, histology provides conclusive evidence. The only therapeutic option available at present is reduction of immunosuppressive therapy.