The principal significance of the urothelial changes caused by polyomavirus activation is in an erroneous diagnosis of urothelial cancer; however, the clue to their benign nature is the smooth structureless nuclear configuration and the relative paucity of affected cells. Though virologic studies and electron microscopy are usually needed to firmly establish the diagnosis, cytology is the most readily available and rapid means of establishing a presumptive diagnosis of human polyomavirus infection. A urine specimen of a 24-year-old man with hemorrhagic cystitis beginning two months after bone marrow transplantation for acute myeloblastic leukemia(M2) was submitted for cytologic evaluation. Cytologic findings revealed a few inclusion-bearing epithelial cells intermingled with erythrocytes, neutrophils, lymphocytes, and macrophages. Most of the inclusion-bearing fells had large, round to ovoid nuclei almost completely filled with homogeneous dark, basophilic inclusion. The chromatin was clumped along the periphery and the cytoplasm was mostly degenerated. The other cells exhibited irregular inclusions attached to the nuclear membrane surrounded by an indistinct halo. These findings were consistent with polyomavirus infection.