5'-DFUR manifests its antitumor activity after being converted to 5-FU by pyrimidine nucleoside phosphorylase (PyNPase), an enzyme having high activity in tumor tissue. We surgically removed tissue samples weighing at least 1 g from 28 patients with gastric cancer and measured the PyNPase activity. The tissue samples were taken from gastric carcinomas, normal gastric tissue, metastasis-positive lymph nodes, metastasis-negative lymph nodes, small intestine, spleen, and other organs. The PyNPase activity was higher in gastric carcinomas than in normal gastric tissue. Similarly, metastasis-positive lymph nodes showed higher PyNPase activity than in metastasis-negative lymph nodes. PyNPase activity in the spleen was approximately equivalent to that in gastric carcinomas. Correlations were assessed between the PyNPase activity of gastric carcinomas and their tissue type. Stage and nutritional status, but there was no correlation with PyNPase activity for any of the variables assessed. The relationship between the PyNPase activity of gastric carcinomas and serum IAP concentration measured within one week before operation was similarly assessed. Mean PyNPase activity in the group of 14 patients with low serum IAP concentrations (< 550 micrograms/ml) was 102.8, while that in 11 patients with high serum IAP concentrations (> or = 550 micrograms/ml) was 145.7. Although this difference was not significant, there tended to be a positive correlation between PyNPase activity and serum IAP concentration (r = 0.74). PyNPase activity in splenic tissue was significantly higher in group of 10 patients with low IAP activity (104.7) (p = 0.020). These results indicated that patients with high serum IAP concentrations, generally associated with a poor prognosis, showed a high PyNPase activity in tumor tissue and splenic tissue. PyNPase activity, including its involvement in the immune response, was thus suggested to be somehow related to prognosis.