We present a patient with pulmonary metastasis from breast cancer who received S-1 (TS-1) and maintained complete response for approximately 10 years after recurrence. A 51-year-old woman underwent modified radical mastectomy for left breast cancer in November 1991. Her cancer was postoperatively classified as pT2 pN0 M0 Stage IIA. As postoperative adjunctive treatment, tamoxifen and hexylcarbamoyl 1-5-FU (HCFU) were given. During the administration period (30 months after surgery), a solitary pulmonary metastasis occurred. Three months after the start of S-1 (100 mg/body/day), the tumor disappeared on images. Thereafter she took S-1 orally for approximately 10 years, and the pulmonary metastatic focus maintained complete response. In addition, no recurrent focus was observed. The adverse events observed during S-1 treatment were nausea, low-grade neutropenia and pigmentation of fingers. All were mild, and S-1 could be continued. Our case illustrates two important characteristics of S-1. First, S-1 was effective even though this patient had a lung metastasis during adjuvant treatment with HCFU. S-1 is a combined formulation containing 5-chloro-2, 4-dihydroxypyrimidine (CDHP; gimestat), which inhibits an enzyme that metabolites 5-FU, dihydropyrimidine dehydrogenase (DPD). Therefore, high 5-FU concentrations are maintained with S-1, and S-1 may be effective in the patients who do not respond to other fluoropyrimidine agents. Second, since S-1 toxicity was mild, long-term treatment for approximately 10 years was possible. Since S-1 contains potassium oxonate (OXO; otastat), gastrointestinal toxicities, the main adverse events of 5-FU agents, could be reduced. The purpose of treatments for metastatic breast cancer is to maintain favorable quality of life (QOL), as well as to improve survival. S-1 could be a valuable agent for breast cancer treatments, since it showed clinical efficacy and mild toxicity, and can be given orally.