Human polyomaviruses BKV and JCV establish a persistent infection in the kidney and possess oncogenic potential in experimental animal models. Their possible involvement in the pathogenesis of human urogenital tumors has been suggested by several studies but is not conclusively resolved. For the present study, highly sensitive quantitative TaqMan PCR assays were developed to detect BKV and JCV DNA sequences in matched pairs of human tumors and surrounding normal tissue. Samples from 55 patients with renal pelvic urothelial carcinomas and from 83 patients with renal cell carcinomas of all histologic subtypes were investigated. Human polyomavirus DNA was detected in renal pelvis samples with a frequency of 16% (BKV) and 15% (JCV), and in kidney samples with a frequency of 1% (BKV) and 23% (JCV), respectively. However, viral sequences were not present more often in tumors than in normal tissue. There was no integration of the potentially oncogenic viral large T-antigen in the tumors, and the viral concentration of both BKV and JCV was not higher in DNA extracted from tumors compared to normal tissue. These data provide no evidence for a causative role of the human polyomaviruses BKV and JCV in the development of tumors of the renal pelvis and the kidney.