Adenomatous polyposis coli (APC) is a rare autosomal dominant disorder in which about 20∼30% of affected individuals do not have a family history. It is characterized by hundreds to thousands of adenomas in the colon. APC usually develops during the second or third decade of life. If the polyposis is not treated surgically, colorectal cancer can develop in almost all patients before age 40. We experienced a case of adenomatous polyposis coli combined with rectal cancer in a 26-year-old male patient complaining of low abdominal pain without a family history of APC. (Korean J Gastrointest Endosc 2003;26:443447)