To elucidate the clinical significance of phenotypic alterations of Lewis antigen in gastric cancer patients, we investigated Lewis antigens by analyzing the genotypes of the Le and Se genes and by comparing the results obtained with the phenotypic expression of Lewis antigen in gastric cancer tissue and blood cells. One hundred and twenty gastric cancer patients were examined and compared with respect to Lewis blood phenotype and genotype. The expression of Le^a, Le^b, sialylated Le^a, and sialylated Le^x antigens was immunohistochemically examined in uninvolved gastric mucosa, intestinal metaplasia, and cancerous tissue. We also analyzed the significance of Lewis antigen expression by analyzing patient survival. The frequencies of the Lewis phenotypes of RBCs corresponding to Le(a+b-), Le(a-b+), and Le(a-b-) were 16%, 58%, and 26%, respectively. The Le and le allele gene frequencies calculated from genotyping in gastric cancer patients were 0.623 and 0.377, respectively. The frequency for Le(a-b-) of the RBC phenotype had a tendency to be higher in cancer patients than in normal healthy Koreans. However, no difference in the Lewis gene frequency was found between these gastric cancer patients and healthy persons. The phenotype of Le(a-b+) was most prevalent in uninvolved gastric mucosal tissue, whereas the most prevalent form in tumor tissue was Le(a-b-). Sialyl-Le^a and sialyl-Le^x antigens were hardly detectable in uninvolved gastric mucosa, whereas the two antigens were expressed highly in intestinal metaplastic mucosa and tumor cells. In conclusion, the loss of Lewis antigen expression in tissue and on RBCs in gastric cancer patients is not a result of genetic influences, but rather a result of sialylation in tissue. We also confirm that poor prognosis is associated with dimeric sialyl-Le^x and vascular spread.