Four patients are reported, to illustrate the wide spectrum of colonic polyps which may occur in multiple form in adults. Included are unusual cases of metaplastic and juvenile polyposis. The subject is reviewed, and the need for accurate histological diagnosis is stressed. The possibility that the difference between familial adenomatous polyposis and Gardner's syndrome may lie merely in the degree of expressivity of the associated extracolonic features is discussed. Surveillance of these patients for peri-ampullary neoplasms in the duodenum is considered. The interrelationship between familial adenomatous polyposis, juvenile polyposis and colorectal cancer is discussed.