e16163 Background: Thymidylate synthase ( TS ) is an important enzyme in de novo DNA synthesis pathway. 5-Fluorouracil ( 5-FU ), an anticancer chemotherapeutic agent used clinically against a variety of cancers including prostate cancer, inhibits DNA synthesis by binding TS. In the present study, we examined TS expression in prostate cancer and investigated its prognostic significance. Methods: Fifty-two prostate cancer tissue specimens were obtained from patients who underwent radical prostatectomy for prostate cancer without neoadjuvant hormonal therapy. Forty-eight prostate cancer tissue specimens were also obtained from patients who underwent radical prostatectomy for prostate cancer with neoadjuvant hormonal therapy. We examined prostate cancer tissue and normal prostate tissue for TS expression by immunohistochemistry. Results: TS was expressed at higher levels in prostate cancer without neoadjuvant hormonal therapy, compared with normal prostate.TS expression in stage T3 prostate cancer was higher than that in stage T2 prostate cancer. In addition, the level of TS expression in Gleason score 7 or greater prostate cancer was higher than that in Gleason score less than 7 prostate cancer. Patients with prostate cancer with negative TS expression without neoadjuvant hormonal therapy had a longer postoperative recurrence-free rate than those with positive expression in the 5 year follow-up. In addition, patients with Gleason score less than 7 prostate cancer with negative TS expression had a much longer postoperative recurrence-free rate than those with positive expression in the 5-year follow-up. TS expression was significantly decreased in prostate cancer patients who received neoadjuvant hormonal therapy, especially stage T2 prostate cancer patients. Conclusions: The current study has demonstrated for the first time that TS expression may be a prognostic parameterr for prostate cancer patients undergoing radical prostatectomy. No significant financial relationships to disclose.