A combined morphologic and histochemical study of metaplastic polyps of the colorectum was undertaken. With increasing size, these lesions showed alterations in architecture and differentiation, increased secretion of carcinoembryonic antigen and a reduced secretion of O-acylated sialomucin. These functional changes are also observed in colorectal cancers, but not in tubular adenomas showing low-grade dysplasia. Since there is little clinical evidence for a metaplastic polyp-carcinoma sequence, it is suggested that the sets of factors which lead to adenoma and metaplastic polyp formation in the colorectum are both required to induce malignant transformation.