The epithelial expression of carbohydrate antigen, stage-specific embryonic antigen 1 (SSEA-1) was examined immunohistochemically in noncancerous specimens from patients with familial polyposis coli, and compared with the colorectal epithelia from patients with sporadic colorectal cancer. In mucosa remote from carcinoma of sporadic cases, SSEA-1 was expressed only faintly in the lower crypts. In mucosa adjacent to carcinoma of sporadic cases, SSEA-1 was expressed not only in the lower crypts but also in the upper crypts. These results corresponded to those observed in the authors' previous study. In the flat mucosa of familial polyposis coli cases, SSEA-1 was detected not only in the lower crypts, but also in both upper crypts and the surface epithelium in contrast with the flat mucosa of sporadic cases. The staining pattern in the upper crypts of the flat mucosa of familial polyposis coli cases was very similar to that of the mucosa adjacent to carcinoma of sporadic cases, but was stronger and more diffuse in the surface epithelium. In microscopic adenomas, SSEA-1 was expressed diffusely. These results demonstrate that the flat mucosa of patients with familial polyposis coli shows preneoplastic changes similar to those in the mucosa adjacent to carcinoma of sporadic cases, and that SSEA-1 is related to adenoma formation in the early stage of carcinogenesis in the colorectum. In addition, the results suggest that immunohistochemical studies of flat mucosa may be useful for the early detection of high-risk individuals in a familial polyposis coli family.