A rare genetic syndrome with an autosomal dominant pattern of inheritance. It was formerly called hereditary flat adenoma syndrome (HFAS) but is now recognized as an attenuated variant of familial adenomatous polyposis (FAP). It is most commonly associated with mutations in the APC gene specifically: exons 1-5, exon 9 and in the distal 3' terminus. Clinical presentation occurs in the middle decades and is characterized by the proximal development of multiple (but less than 100), non-polypoid colonic adenomas. Clinical progression to colorectal cancer may occur later in life but the risk is less than with classic FAP.