Low-dose therapy consisting of cisplatin plus 5-fluorouracil was given to 20 patients with advanced gastric cancer, and specimens were obtained to evaluate levels of 5-fluorodeoxyuridine (FdUMP) and thymidylate synthase (TS), indices of DNA, and proliferating nuclear cell antigen (PCNA). There was a significant correlation between the levels of FdUMP, total TS and free TS in cancer and in normal gastric mucosa, respectively. Total TS of cancer was higher than that of normal tissue, at 5.3 +/- 4.8 and 3.4 +/- 2.2 pmol/g, respectively. On the other hand, there was no relationship nor difference between The TSIR ratio of cancer and normal mucosa. The FdUMP levels of far advanced cancer showed a tendency to be lower than those of the less advanced one, especially in liver metastasis. The total TS was higher in intestinal type and in INF alpha or beta. The free TS was higher in invasive type, liver metastasis and curability C. The TSIR ratio showed a tendency to be lower in far advanced cases, such as invasive type and INF gamma. The correlation between DNA index and TS values and between PCNA labeling index and TS values were good. These results suggest that TS levels would be a predictor for malignant potential as well as for chemosensitivity.