Expression of sialyl-Lewisx (sLex) antigen was studied immunohistochemically in 110 resected human gastric carcinomas using an anti-sLex monoclonal antibody. Lymph node, liver, and peritoneal metastases were clearly more prevalent in tumors expressing high levels of sLex than in those with no or low-level sLex expression. No correlation was found between sLex expression and histologic grade or histologic type of the Lauren classification. Among the tumors with lymph node metastasis, 44% expressed high levels of sLex in both the primary tumor and involved lymph nodes, and 14% of the metastatic lesions demonstrated increased sLex expression. The 5-year survival rate of the patients undergoing complete (R0) gastric resections was 60% in the sLex high-expression group, which was significantly lower than that of the sLex low-expression group (81%) and of the no-expression group (87%) (p < 0.05). These results suggest that high-level sLex expression is related to both an increased risk of metastasis and poor prognosis in gastric cancer patients.