The expression of blood group-related antigens of the Lewis system in normal gastric mucosa, intestinal metaplasia, gastric adenoma, and gastric carcinoma was examined. Ninety-five percent of normal foveolar epithelial samples stained positive for Lewis(b) antigen, whereas only 10.0% expressed Lewis(a) antigen. In contrast, intestinal metaplasia specimens had increased Lewis(a) antigen expression and slightly decreased Lewis(b) antigen expression. A similar pattern of Lewis(a) and Lewis(b) expression was observed in gastric adenomas and intestinal type adenocarcinomas. Lewis(x) and Lewis(y) were detected in all normal deep glands, but were not expressed in the majority of intestinal metaplasia specimens. In addition, only 20-40% of gastric adenomas and gastric carcinomas expressed Lewis(x) and Lewis(y) antigens. These changes in Lewis antigen expression in intestinal metaplasia, adenomas, and intestinal type adenocarcinomas suggest that altered expression of Lewis blood group-related antigens may correlate with cell transformation processes.