De novo carcinoma of the lower urinary tract in renal allograft recipients. Immunocompetence has been postulated as an important defense against the progression of urothelial carcinoma. Three cases of de novo lower urinary tract carcinoma in renal transplant recipients demonstrated the potential for unusually rapid urothelial extension and invasion in chronically immunosuppressed patients. Two patients had a history of perineal condyloma acuminata; tumors from 1 of these harbored the genetic sequences of human papillomavirus type 6. One patient had multiple manifestations of cyclophosphamide-related urothelial injury, including bladder carcinoma. Treatment of 2 patients culminated in a radical operation during which the remaining native urinary system was resected completely, with sacrifice of the allograft kidney in 1 and diversion into an ileal conduit in 1. The remaining patient underwent urethrectomy and partial cystectomy with a sigmoid conduit. 