Background.Viral (v) interleukin (IL)-10 is expressed by Epstein-Barr virus(EBV) and has pro- and anti-inflammatory actions similar to human IL-10. EBV is also a known factor in the development of posttransplant lymphoproliferative disorder (PTLPD) in allograft recipients. We observed a patient with widespread PTLPD 9 months after renal transplantation, who subsequently maintained renal function despite minimal immunosuppression, and we investigated a possible link between these factors.Methods.The patient's chart was reviewed for relevant history. EBV DNA in blood and tissues was assessed by polymerase chain reaction. Human and vIL-10 and γ-interferon mRNA were evaluated with reverse transcription-polymerase chain reaction using nested primers.Results.After the diagnosis of PTLPD, the patient was maintained on prednisone (8 mg/day) as the only immunosuppression with preserved renal function for 17 months until death as a result of pulmonary failure. She had continuously high blood levels of EBV DNA, although only mild persistent intrarenal atypical lymphocytic infiltrates. Human IL-10 mRNA was never present; in contrast, intragraft vIL-10 mRNA was identified and associated with resolution of an intervening episode of severe acute transplant rejection.Conclusions.We suggest that the preserved renal function resulted from the anti-inflammatory actions of vIL-10 inhibiting acute rejection in the renal allograft.