Of 636 polyps removed during endoscopy between 1973 and 1975 at the University Hospital, Zurich, 36 (5.5%) were of the hyperplastic and 18 (2.8%) of the juvenile type. One polyp was seen in a female patient with Peutz-Jeghers syndrome. The vast majority of the polyps (581, 91.5%) were neoplastic in origin; 70% were tubular adenomas, 16% villous adenomas, and 14% intermediate forms or tubulo-villous adenomas. On the basis of a continuous spectrum in histologic structure and similar cellular dedifferentiation, these three types of adenoma may be viewed as different forms in the development of the same neoplastic process. Hyperplastic and hamartomatous polyps are innocuous, whereas the neoplastic forms may well turn aggressive. Malignant change was observed in 5.1% of our material, particularly in villous adenomas and in polyps exceeding 1 cm in diameter. The presence or absence of invasive growth through the muscularis mucosae is of prime importance for therapy. In accordance with WHO nomenclature, the term carcinoma is used only in the presence of such infiltration. If, in addition, the tumor tissue is not well differentiated, additional segmental resection may be required. The term "focal carcinoma" is no longer in use and has been replaced by "severe focal atypia". In these cases, primary polypectomy for diagnostic purposes is also the optimal therapy, and is as effective here as in cases of benign adenoma.