We report a patient with rectosigmoidal adenomatous polyposis.A 57-year-old male presented with a submucosally invasive well-differentiated adenocarcinoma in the rectum and approximately 100 adenomatous polyps with an extremely unusual distribution limited exclusively to the rectum and sigmoid colon.There was no family history of colorectal disease or any related disorders. No extracolonic manifestations were found. Because this case was considered to be a discriminative phenotype of familial adenomatous polyposis, DNA from a peripheral sample of whole blood was screened for APC germline mutation by a combination of protein truncation test and single stranded conformation polymorphism, but no mutation was found.This patient may have a novel entity of adenomatous polyposis with a peculiar distribution. It may be caused by some genetic alteration other than APC mutation.