Both clinical and experimental observations establish that an adenocarcinoma of the colon is likely to occur at the suture line of ureterosigmoidostomy. The carcinogenesis depends on the initial presence of urine, feces, urothelium, and colonic epithelium in close apposition at a healing suture line. It does not occur in isolated colon loops used for urinary diversion. In our rat model, tumors were completely prevented by interposing ileum between the urothelium and colon. Clinical prevention requires that accurate hospital registries of patients at risk be established and that repeated annual colonoscopy be carried out on all of them.