A 43-year-old woman with end-stage renal disease originating from IgA nephropathy entered chronic haemodialysis therapy. She then received an ABO-incompatible living related renal transplantation. Initial immunosuppression consisted of azathioprine, methylprednisolone and tacrolimus. At 155 days after transplantation, the azathioprine was changed to mycophenolate mofetil for continuous graft dysfunction. Furthermore, a total of three courses of anti-rejection therapy was given. At 665 days after transplantation, diagnosis of BK-virus nephropathy was made by immunohistochemical analysis and viral DNA assay. Therefore the immunosuppression therapy was reduced for graft dysfunction. All five renal biopsy specimens were examined retrospectively in order to determine when the BK virus nephropathy had developed. The expressions of SV40 large T antigens were detected from the third (117 days) to the fifth (665 days) biopsies, with increasing numbers of SV40 large T antigen positive cells. In addition, many cells contained inclusion bodies which were already present in the urinary sediment for 3 months post-transplantation. Although it is difficult to make a diagnosis of early stage of BKVN, we have to consider with caution if urinary cells with inclusion body are seen. Awareness of BKVN at the earliest opportunity is important in order to avoid over-immunosuppression.