Thymidine phosphorylase (TP) converts 5'-deoxy-5-fluorouridine (5'-DFUR, doxifluridine), an intermediate metabolite of capecitabine, to 5-fluorouracil (5-FU). While dihydropyrimidine dehydrogenase (DPD) catalyzes 5-FU to inactive molecules. We investigated TP and DPD levels in tumor tissue to assess their clinical significance as indicators for selecting colorectal cancer patients for 5'-DFUR adjuvant chemotherapy. A total of 88 colorectal cancer patients were classified into Dukes' B and C groups and treated for 2 years with oral 5'-DFUR (800 mg/body/day). During the follow-up period, 20 of the 88 patients developed a recurrence. All the patients were examined retrospectively for primary tumor TP and DPD levels and clinical response to 5'-DFUR. Results showed that: a) median levels of TP and DPD in the primary tumor, measured by enzyme-linked immunosorbent assay (ELISA), were, respectively, 43.6 and 32.3 U/mg protein. Primary tumor TP levels of the 20 patients who had a recurrence were lower than those of the 68 patients with no recurrence (p=0.07); b) although there were no significant differences in clinicopathologic features between high and low median TP level groups, disease-free survival was better in the high TP than in the low TP group (89% vs. 64%, at year 4); and c) of patients classified into 4 groups such as high TP/DPD, high TP but low DPD, low TP but high DPD, and low TP/DPD, patients with high TP but low DPD had the best disease-free survival, whereas the low TP but high DPD group had the worst survival. These results suggest that TP and DPD levels in primary colorectal tumors may be a useful indicator for selecting patients likely to respond to 5'-DFUR adjuvant chemotherapy and probably capecitabine, a prodrug of 5'-DFUR.