Pyrimidine nucleoside phosphorylase (PyNPase) activity was measured in gastric cancer tissue from 25 patients who underwent resections of gastric cancer. The relation between the activity and host and tumor factors in gastric cancer was studied, and the following results were obtained. 1. PyNPase activity was 128.3 +/- 99.5 in cancer tissue and 37.2 +/- 23.1 in non-cancer tissue. The level was significantly higher in cancer tissue (p < 0.0001). 2. With respect to host factors, the PyNPase activity tended to be high in patients in whom cell-mediated immune response was maintained. 3. With respect to tumor factors, the values tended to be high in patients who were positive for lymph vessel and venous invasion, and positive for lymph node metastasis. 5'-deoxy-5-fluorouridine(5'-DFUR) is an anticancer agent which manifests antitumor effects when it is transformed into 5-FU by PyNPase. When this agent is administered to gastric cancer patients, it can be expected to be more effective in the above types of patients because of its characteristics.