Colonic adenocarcinoma has been frequently reported after ureterosigmoidostomy based on carcinogenic substances created when feces and urine are mixed. However, colonic adenocarcinoma has never been reported arising in an Indiana pouch after cystectomy. We report a case of adenocarcinoma arising in a urinary pouch after cystectomy for transitional cell cancer. We believe this to be caused by hereditary nonpolyposis colon cancer (an autosomal dominant syndrome that puts individuals at risk for genitourinary, colonic, and several other cancers) rather than carcinogenic effects of urine on colonic mucosa. When planning urinary reconstruction after cystectomy for malignancy, it is important to consider the possibility that an individual may suffer from hereditary nonpolyposis colon cancer before selecting the colon as a urinary reservoir.