Sialylated Lewis (S-Lex) has been studied histologically and serologically in stomach cancer by the CSLEX1 monoclonal antibody. S-Lex was expressed in 73.9% of 46 stomach cancer tissues, 29.4% of metaplastic parts adjacent to cancer, and none of six gastric ulcer tissues including metaplasia. Serologically positive percentages were as follows: 26.0% of 100 stomach cancers, 0.9% of 322 benign diseases, and 0.7% of 280 healthy donors in the sera, as well as 72.4% of 29 ascites of stomach cancers and 5.3% of 17 effusions of benign diseases. These findings demonstrate that S-Lex possesses a potential usefulness as a tumor marker in stomach cancer.