We have seen the re-emergence of a virus, normally latent in the urinary tract, which becomes activated in renal transplant patients due to potent immunosuppression. Polyomavirus reactivates within the allograft kidney, causing renal dysfunction and graft loss in 40 to70% of patients with overt renal dysfunction and histologically proven Polyomavirus nephropathy. More research and data are needed to further elucidate the pathogenicity of the virus, and to find an effective antiviral agent.