SUMMARY Human polyomavirus (BK) was detected in two renal allograft recipients as a result of routine examination of Papanicolaou-stained smears of urinary sediment in the light microscope. Infection with this recently identified virus was confirmed by virus isolation and electron microscopy. The cytological, histological, and ultrastructural changes due to the virus are described, and virus excretion is correlated with the clinical progress of the patients and the pathological findings. The transplant ureters in both patients were found to be ulcerated and stenosed, and virus-infected cells were observed in the ureteric epithelium. We suggest that the administration of high-dose steroids in transplantation may permit active infection with human polyomavirus to occur in ureteric epithelium which has been damaged by ischaemia or inflammation.Infection with human polyomavirus (BK) was first described by Gardner et al. (1971), who isolated the virus from the urinary tract of a renal allograft recipient. The patient (BK) developed clinical evidence of ureteric obstruction coincident with the virus infection, and at laparotomy a segment ofdonor ureter was found to be ulcerated and stenosed. Since this initial observation was made, serological studies have shown that reactivation of latent BK virus is common in renal allograft recipients (Coleman et al., 1973b) and polyomavirus particles have been observed in the urine of 44% of transplant patients (Lecatsas et al., 1973); but, despite the frequency of infection, there have been no further case reports of BK virus infection associated with ureteric stenosis or obstruction.In this paper we report pathological changes in two patients who were found to have active infection with human polyomavirus (BK) after renal transplantation. BK virus infection was suspected when numerous large inclusion-bearing cells were seen in routine Papanicolaou smears of the urinary sediment, and the cytological diagnosis was confirmed by electron microscopy and virus isolation. Examination of postmortem material and surgical specimens revealed that, in both cases, BK virus infection was associated with narrowing, fibrosis, and ulceration of the donor ureter.Received for publication 20 October 1977 Special methods of investigation CYTOLOGY Ten millilitres of freshly voided urine were concentrated by centrifugation and the deposit was resuspended in 1 ml of the supernatant urine. Three slides were made from the cell suspension by cytocentrifugation (Shandon-Elliot). Two slides were stained by the Papanicolaou method and the third by methyl green pyronin. The slides were screened by light microscopy for activated lymphocytes, inclusionbearing cells, renal tubular cells, and casts. ELECTRON MICROSCOPYTen millilitres of urine were concentrated by centrifugation and the deposit was resuspended in 0 5 ml of supernatant placed in a conical polyethylene embedding capsule and centrifuged again. The supernatant in the capsule was discarded, leaving a button of cells in the point of the capsule. A 1 ...