The development of malignancy in a renal transplant graft is an uncommon phenomenon. A renal neoplasm developing in the adult donor kidney of a pediatric transplant recipient has only rarely been reported. We report a case of collecting duct carcinoma arising in association with BK virus nephropathy in an adult living-related donor renal allograft to a pediatric recipient. Our case is the second report of neoplasia occurring in association with BK virus nephropathy post-transplantation, suggesting that BK virus may play a role in oncogenesis. It has been proposed that the T-Ag protein encoded by the polyomavirus family of viruses disrupts chromosomal integrity, creating oncogenes, and inactivating tumor suppressor genes. In our study, immunohistochemical staining with antibody directed against BK virus large T antigen showed nuclear staining within urothelium, tubular epithelium, tubular intraepithelial neoplasia, and invasive carcinoma. In situ hybridization did not identify BK virus DNA within neoplastic cells. T-Ag protein expression has been shown to be tumor-specific in bladder, gastric, and colorectal cancers. The finding of T-Ag protein expression in both intraepithelial and invasive neoplastic tissues in our case raises the possibility of BK virus as a causative agent in oncogenesis.