Changes in gastric mucosa that antedate gastric carcinoma. Endoscopic biopsy specimens of the gastric mucosa from 13 patients who were found at follow-up examination to have gastric carcinoma were compared for abnormal histologic features, type of intestinal metaplasia, and presence of immunoreactive carcinoembryonic antigen (CEA), with specimens from 40 tumor-free controls. Villus-like changes and angular infolding, cytologic nuclear pleomorphism, distinct nuclear border, irregular thickness of the nuclear membrane, irregular chromatin clumping, prominent nucleoli, and distinct nucleoli were manifestations of the carcinoma group. Angular infolding, distinct nuclear border, irregular thickness of the nuclear membrane, and distinct nucleoli were also observed in the latent stage before detection of carcinoma. The individual features, however, lacked specificity. Histochemically, a IIB subtype of intestinal metaplasia, and immunoreactive CEA in the cytoplasm of foveolar epithelium appeared exclusively in the patients with carcinoma. These findings indicate that the gastric epithelium of patients with gastric carcinoma tends to be morphologically and histochemically abnormal even before the recognition of classical dysplasia. This can be described as abnormal epithelium and is believed to provide the soil on which gastric carcinoma develops. 