Thomsen-Friedenreich antigen (T antigen) has been supposed to be a cancer-specific carbohydrate antigen. We have previously shown that one third of the Japanese population normally expressed T antigen in gastric surface epithelia and the other two thirds expressed fucosyl-T antigen. Their sialylation and blocked-synthesis were associated with diseased conditions. In the present study, we studied gastric surface epithelial expression of monosaccharide antigen Tn, i.e., a precursor of T antigen, and sialyl-Tn. Normal fundic and pyloric epithelia, respectively, expressed Tn supranucleally and cytoplasmically, but did not express sialyl-Tn. In the intestinal metaplasias and intestinal-type adenomas, goblet cells expressed sialyl-Tn in their vacuoles, and absorptive cells expressed Tn apically. In gastric-type adenomas, Tn, but not sialyl-Tn, was detected. Intestinal-type cancers expressed Tn and sialyl-Tn more often than the diffuse-type cancers. Five cancers did not express Tn, sialyl-Tn, or the T-related antigens. In these, four were diffuse-type cancers. We concluded that: a) normal gastric epithelial cells express Tn; b) metaplastic differentiation is associated with sialylation of Tn and c) expression of Tn and sialyl-Tn is depressed in the gastric cancers.