We have found that elevation of preoperative serum sialyl-Tn antigen (STN) levels is associated with a poor prognosis for gastric cancer patients, and these high levels remain in the advanced stage of the disease. We have now examined findings with the combined assay of STN and carcinoembryonic antigen (CEA) levels with regard to prediction of the prognosis of gastric cancer patients. Serum CEA levels and STN levels were determined preoperatively in 349 Japanese patients with gastric cancer. The patients were divided into four groups: (A) the CEA (-) STN (-) group (CEA < or = 5 ng ml-1, STN < or = 45 U ml-1, n = 286); (B) the CEA (-) STN (+) group (CEA < or = 5 ng ml-1, STN > 45 U ml-1, n = 31); (C) the CEA (+) STN (-) group (CEA > 5 ng ml-1, STN < or = 45 U ml-1, n = 17); and (D) the CEA (+) STN (+) group (CEA > 5 ng ml-1, STN > 45 U ml-1, n = 15). Clinicopathological features and the prognosis of these groups were examined. The distribution of two markers showed no significant correlation. The patients in the CEA (+) STN (+) group (group D) had more advanced disease than the patients in CEA (-) STN (-) group (group A); tumour size was larger, serosal invasion was prominent, lymphatic and vascular involvement was frequent and the tumour was more infiltrative. Lymph node metastasis and hepatic metastasis were more common. Total gastrectomy was usually performed, and the non-curative rate was higher. The 5-year survival of patients in the CEA (+) STN (+) (group D) was 14.5 +/- 9.5%, that is lower than that of patients in any other group [CEA (+) STN (-) (group C) 44.1 +/- 12.7% (P < 0.05); CEA (-) STN (+) (group B) 60.1 +/- 9.5% (P > 0.05); CEA (-) STN (-) (group A) 77.6 +/- 9.5% (P < 0.05)]. This combined assay of these markers will aid in estimating the prognosis and selecting appropriate drugs and care for gastric cancer patients.