To assess the efficacy of 5'-DFUR, an intermediate of capecitabine, for adjuvant treatment of early breast cancer, we conducted an open-labeled multi-center randomized controlled trial to compare postoperative 5'-DFUR treatment with surgery alone. We enrolled 1217 primary breast cancer patients and randomly assigned them into two treatment groups; one received six-month postoperative 5'-DFUR treatment by consecutive or intermittent administration, and the other surgery alone. Follow-up surveys were conducted once a year for all subjects simultaneously and examined their outcome/presence or absence of the cancer recurrence. The central study committee reviewed all follow-up data and judged the recurrence data to be used for the analysis. Eight-year follow-up data showed no significant differences in relapse-free and overall survival between the two groups, and 5'-DFUR treatment regimen showed an extremely high tolerance. Possible explanations are discussed for the finding of no significant survival difference between adjuvant 6-month 5'-DFUR monotherapy and surgery alone in early breast cancer.