This randomized controlled trial will compare oral 5-fluorouracil derivatives, TS-1, with intravenous standard chemotherapy such as taxanes in women with metastatic or recurrent breast cancer. Patients with hormone-resistant breast cancer are assigned to either TS-1 (40–60 mg twice daily for 28 consecutive days, followed by a 14-day rest period) or standard chemotherapy (docetaxel 60–75 mg/m² at 3- or 4-week intervals, paclitaxel 175 mg/m² at 3- or 4-week intervals or paclitaxel 80–100 mg/m² weekly, followed by a 1-week rest period). Treatment will be repeated until tumor progression or ≥4 courses for TS-1 and ≥6 courses for taxanes. The primary endpoint is overall survival. Secondary endpoints are progression-free survival, time to treatment failure, adverse events, health-related quality of life and cost-effectiveness. A threshold hazard ratio of 1.333 will be used to determine whether overall survival in the TS-1 group is equivalent (not inferior) to that in the taxane group. The target number of registered patients is 600.