PURPOSE: Thymidylate synthase (TS) is a critical enzyme in the DNA synthesis and an important target of cancer chemotherapeutic agents, such as 5-fluorouracil (5-FU). Recent studies suggest that TS expression is related to the prognosis of various cancers and the mechanism of chmotherapeutic drug resistance. This retrospective study was performed to determine whether TS expressions in primary colorectal tumors influence the overall survival and recurrence for patients with colorectal cancer. METHODS: Intratumoral TS expression was evaluated by immunohistochemical staining using TS-106 monoclonal antibody in primary colorectal cancers of 64 patients who had undergone surgery from July, 1995 to June, 1999. The relationship between TS expressions and patients' survival was evaluated statistically. The median follow-up period was 25.7 months. RESULTS: Overall positive TS expression rate was relatively high (54.7%) in colorectal cancers, and overall disease-free survival rate was significantly higher in the TS positive group (P=0.0204). But there was no statistically significant differences in overall survival rates (P=0.249) and tumor recurrence rates (P=0.732) between positive TS group and negative TS group. CONCLUSIONS: These results suggest that TS expression status in the colorectal cancer tissue is only related to the overall disease-free survival rates, not the overall survival rates and tumor recurrence rates. More objective method and long term follow up study will be required for accurate assessment of clinical importance of TS expression in colorectal cancers.