BACKGROUND Oncofetal antigen 5T4 is defined by a monoclonal antibody raised against placental trophoblast membranes. Antigen 5T4 is reported to be expressed only on trophoblasts and neoplastic cells and would be a potentially useful marker for neoplastic cells. MATERIALS AND METHODS We evaluated the expression of 5T4 oncofetal antigen in gastric cancer by immunohistochemistry and investigated whether or not the expression of this antigen is correlated with the prognosis of patients with gastric cancer. RESULTS The 5T4 antigen was detected in 51.6% of primary gastric cancer (n = 62) and 5T4-reactivity was more frequently observed in carcinoma with lymph node metastasis (p < 0.05). The overall survival of 5T4-positive patients was significantly worse than that of 5T4-negative patients, analyzed by the log-rank test (p < 0.05), although multivariate analysis indicated that 5T4 expression was not an independent prognostic factor. CONCLUSION The assessment of 5T4 expression may be useful in identifying gastric cancer patients with a poor prognosis.