S-1 is a novel oral anticancer agent consisting of tegafur, a prodrug of 5-fluorouracil, and 2 modulators. A phase I study of sequential S-1 and cyclophosphamide(CPA)therapy was conducted to determine the dose-limiting toxicities(DLTs)and recommended doses(RDs)in patients with metastatic or recurrent breast cancer(MBC). Patients with MBC received sequential S-1 and CPA. Chemotherapy consisted of administration of S-1 twice daily on days 1-14 at escalating doses of 40, 50, 65, and 80mg/m2/day and CPA at 100 mg/body/day on days 15-28. The schedule was repeated twice at a 4-week interval. The purposes of this study were to determine the RDs, safety, and efficacy of the regimen. A total of 12 patients were registered. No patients experienced DLTs, and the RDs of S-1 and CPA were 80mg/m2/day and 100 mg/body/day, respectively. The response rate was 50. 0%. In conclusion, sequential therapy with S-1 and CPA could be safely and effectively used for the treatment of MBC, and the RDs for this regimen were determined to be 80mg/m2/day for S-1 and 100 mg/m2/day for CPA.