A retrospective survey of 278 patients who underwent urinary diversion for benign disease has revealed an excess risk factor for cancer at the ureterocolic anastomosis of approximately 100 fold. No malignancies were detected in isolated ileal or colon conduits, although recently a few such cases have been reported. Coupling this experience with a review of the literature, it would appear that the tumour is of colonic origin, adenoma and adenocarcinoma occurring at, or close to, the ureteric implant into the colon after a long latency of approximately 25 years, often in young patients. Nitrate reducing bacteria and N.nitroso compounds have been demonstrated in rectal urines of patients with ureterocolic anastomoses and also in some isolated loop urines. In fact, N.nitrosation and, possibly, carcinogenesis appears to depend not so much upon the kind of diversion, but rather the presence or absence of a mixed bacterial flora in the urine.