BACKGROUND Polyomavirus associated nephropathy (PVN) in renal transplant recipients has been observed with increasing frequency recently and has emerged as a cause of allograft failure linked to highly potent new immunosuppressive regimens containing tacrolimus or mycophenolate mofetil (MMF). METHODS Polyomavirus associated nephropathy was identified in nine out of 182 patients who received renal transplantation between October 1998 and July 2003. PVN was confirmed by allograft biopsy. The clinical records of these nine patients were reviewed, as were all of the allograft biopsies. Electron microscopy was performed in all nine cases. After the diagnosis of PVN, maintenance immunosuppression was reduced. The clinical course and outcome of the PVN patients were reviewed in relation to manipulation of immunosuppressive agents. RESULTS There were nine cases of PVN in renal transplant recipients and the incidence of PVN was 4.9%. All patients with PVN were under triple immunosuppression comprising tacrolimus and MMF. The mean time to a diagnosis of PVN was 7.8 months after transplantation. Three of the nine patients received antirejection therapy prior to PVN. Seven out of nine PVN patients presenting acute allograft dysfunction were initially treated with high-dose intravenous steroid pulse or OKT3 before reduction of the immunosuppression. After reduction of the immunosuppression, seven patients stabilized their renal function. Two (22%) lost their grafts due to persistent PVN and chronic rejection. Two (22%) patients later developed acute rejection after reduction of the immunosuppression. CONCLUSIONS PVN can cause allograft dysfunction and graft loss. Renal allograft recipients who are at risk of PVN should be routinely screened with urine cytology and quantitative measurements of viral load in the blood, particularly patients who had graft dysfunction. Early diagnosis and judicious alteration of immunosuppressive agents might permit a superior prognosis and reduce the graft loss from PVN in renal transplant recipients.