Polyoma virus nephritis is increasingly being recognized as a cause of allograft dysfunction in renal transplant patients. We describe and illustrate the findings in a renal biopsy and a concurrent urine sample from a patient with polyoma virus nephritis. The urine cytology findings were highly suggestive of a tumor due to the presence of large number of atypical cells with marked hyperchromasia, high nuclear/cytoplasmic ratio in a necrotic background. Scattered cells with viral cytopathic changes were also identified, therefore indicating an infectious process with associated reactive cellular changes rather than a tumor. The cytological findings could thus be explained by the viral cytopathic and cytolytic changes, as well as marked reactive and degenerative changes in tubular epithelial cells. Int J Surg Pathol 8(4):329-331, 2000