Expression of sialyl-Tn antigen (STN) was examined by an immunohistochemical method in 85 primary gastric carcinomas. The STN expression occurred in 53 (62.4%) cancers, and the positive staining was correlated with degree of gastric wall and lymph vessel invasion, lymph node metastasis, and stage of tumour. Five-year survival rates of patients with STN-positive cancers (47.2%) were significantly lower than those with STN-negative cancers (84.4%) (P < 0.01), and patients with STN-positive cancers at stage III and stage IV had a worse prognosis. In the cancers with serosal invasion, patients with STN-positive cancer disclosed a significantly poorer prognosis than those with STN-negative cancers (P < 0.01). Therefore, it is suggested that a careful follow-up study and intensive postoperative therapy are needed for patients with advanced gastric cancers with positive STN expression.