The prognostic value of the immunohistochemical expression of Sialyl Tn antigen (STn) was evaluated in 242 patients with gastric carcinoma. Formalin-fixed, paraffin-embedded specimens of gastric adenocarcinomas were stained with the monoclonal antibody C1282, produced by immunization with ovine submaxillary mucin (OSM). Positive immunoreactivity for STn was observed in 149 (62%) patients. The expression of STn did not correlate with stage of disease (TNM), tumour location, presence of lymph-node or distant metastases, histological type, age or gender. STn immunoreactivity correlated strongly with overall survival in univariate analysis. The median survival in the STn-positive group was 21 months, in comparison to 38 months in the STn-negative group. The difference in survival between STn-negative and STn-positive tumours was significant in patients with stage-I cancer, but not in patients with stage-II -III or -IV disease. STn immunoreactivity emerged as an independent prognostic factor in Cox multivariate analysis. It is concluded that the immunohistochemical expression of STn is a good marker in the prediction of survival in patients with stage-I gastric carcinoma.