Serum levels of tumor markers almost not detectable in precancerous states or early cancer, the behavior of the tumor associated antigen CA 19-9 in colorectal carcinogenesis was analysed. In order to investigate the adenoma-carcinoma sequence, tissue antigenic expression was measured in adenomas, characterized by different size and histology, and compared with that found in normal colonic mucosa. Tissue content of CA 19-9 was determined by an immunoperoxidase technique (ABC-POD) in 88 colonic polyps, and correlated to the degree of histological dysplasia, and dimension of adenoma. Tissue content of CA 19-9 was also evaluated in non-adenomatous mucosa obtained by endoscopic biopsy. Among the 88 polyps, 50 showed the tubular histological type, while 31 resulted tubulovillous and 7 villous. High degree of dysplasia was present in 7 adenomas (7.9%) and focal carcinoma was observed in 6 (6.8%). Positivity for CA 19-9 was registered in 60.2% of adenomas. No correlation was found between tissue-CA 19-9 and degree of dysplasia, size of adenoma and villous component. However, a statistically significant correlation was observed between expression and cellular distribution of the antigen (chi 2 = 98.07, p < 0.00001). Our data confirmed CA 19-9 expression in adenomas, but it is unlikely this tissue antigen proves to be a reliable marker of adenoma-carcinoma sequence.