The pathogenesis of juvenile polyps of the colon was studied in a patient with multiple juvenile polyposis who underwent proctocolectomy for rectal carcinoma and antrectomy for associated polyps of the stomach. Numerous polyps up to 3 cm in diameter were present predominantly in the cecum and rectum, and in addition there was an adenocarcinoma in the rectum. Microscopically there were five categories of lesions: 1) Hyperplastic epithelial foci and small hyperplastic polyps; 2) Typical Juvenile polyps; 3) Juvenile polyps with focal adenomatous epithelium; 4) Adenomas; and 5) and adenocarcinoma. The five categories could represent a pathogenetic sequence, beginning with epithelial hyperplasia, leading to small hyperplastic polyps which become inflamed and enlarge, forming juvenile polyps. Focal adenomatous areas which develop in some juvenile polyps might give rise to adenomas and in turn lead to carcinoma. Although juvenile polyps are generally not considered to be premalignant lesions, this case demonstrates that neoplastic changes may occur in juvenile polyps in certain individuals, and raises the possibility that these may on occasion give rise to carcinoma.