Detailed histologic analyses have been performed in 75, minute, colorectal polyps from familial polyposis patients. All polyps were composed of typical adenomatous tissue, with, in addition, some normal-looking glands still to be found among the neoplastic elements. Bifurcating glands were not observed, and branching patterns were present in 27% of the polyps only. In 86% of the lesions, the number of gland openings along the polyp surface was larger than the number of gland bases observed along the muscularis mucosae. This difference increased with polyp size (r = 0.9043). Those data, together with previous radioautographic observations, suggest that formation of new adenomatous glands mainly results from an infolding of the surface epithelium between normal, preexisting glands. This mechanism sharply contrasts with villous polyps in which papillary projections arise upwards from the mucosal surface.