Background: Treatment for metastatic breast cancer (MBC) in patients who have relapsed from anthracycline and taxane is difficult. S-1, an oral 5-FU derivative, has demonstrated a potential antitumor effect in patients with MBC. Thus, we evaluated the efficacy and safety of S-1 as second-line chemotherapy MBC patients in a phase II trial. Methods: The study was conducted at seven centers in China and enrolled MBC patients who had previously relapsed from one chemotherapy regimen. The median progression-free survival (PFS) was the primary end point. The treatment schedule involved the administration of S-1 at a standard dose based on the body surface area (BSA) in 28-day cycles with consecutive administration followed by a 14-day rest, as follows: 40 mg twice daily if BSA < 1.25 m(2); 50 mg twice daily if 1.25 m(2) <= BSA <= 1.5 m(2); and 60 mg twice daily if BSA > 1.5 m(2). Results: Thirty-three patients were included in the analysis. S-1 demonstrated moderate efficacy with a PFS of 3.3 months, a response rate of 33.3%, and a disease control rate of 72.7%. The treatment was well-tolerated with mild-to-moderate toxicity. Grade 3 adverse events (AEs) occurred in 4 patients (2 with hyperbilirubinemia, 1 with anorexia, and 1 with vomiting). Grade 4 AEs were not observed. Conclusion: S-1 demonstrated encouraging efficacy and safety in a prospective trial as second-line treatment in MBC patients. All AEs were manageable; however, bilirubin monitoring is recommended during treatment.