AIM: Reactivation of latent BK virus in kidney-transplanted patients results in severe graft dysfunction. The role of retroviruses infecting also latently target cells is not investigated so far in this setting. We determined the presence or induction of retroviruses in sera of immunosuppressed patients with renal allografts at the timepoint of organ rejection or ongoing polyomavirus nephropathy. PATIENTS AND METHODS: Sera of patients with acute kidney rejection or polyomavirus nephropathy (n=25) and controls (n=8) were tested for reverse transcriptase activity by the ultrasensitive product enhanced reverse transcriptase (PERT) assay. In parallel, kidney biopsies were investigated for histological signs of kidney rejection or polyomavirus nephropathy confirmed by either immunofluorescence or immunohistochemistry. RESULTS: None of the investigated sera, specifically those of patients with ongoing BK virus nephropathy, indicated reverse transcriptase activity. CONCLUSION: Our results do not support the idea of the induction of known or unknown retroviruses in patients with kidney transplantation, even under highly immunosuppressive therapies.