Expression of thymidylate synthase (TS) has been studied as a prognostic factor and mechanism of drug resistance in gastric cancers. The relationship between TS expression in surgically resected specimens and clinicopathological factors was examined in 216 gastric cancer patients. Immunohistochemical demonstration of the protein was achieved using an anti-TS polyclonal antibody. Positive TS staining was observed in 50 patients (23.1%). Lymph node metastasis was more frequent in patients with TS-positive tumors than in those with TS-negative tumors (P<0.01). Patients were followed for more than 5 years and survival was examined. In 163 patients who received fluorouracil (FU)-based chemotherapy, the overall 5-year survival rate was 41.8% for patients with TS-positive tumors and 57.0% for patients with TS-negative tumors (P<0.01). In the 53 patients who did not receive chemotherapy, these figures were 25.6% and 79.5%, respectively (P<0.05). In patients with T3 gastric cancer who were treated with curative gastrectomy, however, FU-based chemotherapy did not affect survival of either patients with TS-positive tumors or with TS-negative tumors. Multivariate analysis also revealed TS expression to be a significant variable for predicting postoperative survival (P<0.05). These results indicate that TS expression can be used as an independent prognostic factor for patients with gastric cancer. However, TS expression is not a major predictor of the efficacy of FU-based chemotherapy.