Background/Aims: Sialosyl-Tn, a mucin-associated carbohydrate antigen, is not expressed by normal mucus-producing cells of the stornach but is expressed in metaplastic, premalignant and malignant gastric tissues. The frequency of STn expression and its prognostic role in gastric cancer are variable. We have examined the relationship between the expression of Sialosyl-Tn (STn) antigen and the clinical characteristics and prognosis of the patients in primary gastric cancers by immunohistochemistry. Methods: The study sample consisted of 127 primary gastric cancers(M:F=2.2:1, mean age;54.7+12.4) that had been surgically resected. Using mouse monoclonal antibody TKH-2(Otsuka Pharmaceutical Co. Ltd., Japan), specific for the STn epitope, we performed immunohistochemical staining by ABC methods. We examined the correlations between the expression of STn antigen and patients age, sex, tumor location, depth of invasion, degree of differentiation, histologic grade, lymph node metastasis, distant metastasis and median survival duration. Results: The overall positivity was 55%. The grade of STn expression was higher in patients with LN metastasis than without LN metastasis(p<0.05). The survival duration was shorter in STn positive group(mean=16.7 months) than STn negative group(mean=24.9months)(p<0.05). The expression of STn was not related to patient age, sex, tumor location, depth of invasion, degree of differetiation, histologic grade, and distant metastasis. Conclusions: The expression of STn by tumor cell was higher in the LN positive group than in the LN negative group. The STn expression of tumor cells may be one of the poor prognostic factors in gastric cancer. (Korean J Gastroenterol 1997; 30:590-596)