In rare cases the diagnosis of Familial Adenomatous Polyposis (FAP) may not be simple or straightforward. We describe the case of a 54-year man in whom endoscopic and histological features of FAP led to proctocolectomy with ileoanal anastomosis. At anatomical examination, air-filled cystic formations in the submucosa and subserosa of the entire large bowel led to the diagnosis of "Pneumatosis Cystoides Intestinalis", a rare clinical condition featured by the infiltration (or the production) of gas within the intestinal wall. In this case the disease was associated with a long-lasting ingestion of acarbose; as suggested by previous reports, it is possible that gas produced by the fermentation of this unabsorbable carbohydrate penetrates the bowel wall giving origin to cystic formation and to the endoscopic appearance of several polypoid lesions in the large bowel. This report indicates that in rare occasions Pneumatosis Intestinalis can lead to a wrong diagnosis of FAP.