We had 3 patients with recurrent/advanced breast cancer in whom a notable reduction in the size of the targeted tumors was seen following administration of S-1 (TS-1), a new oral pyrimidine fluoride anticancer drug. All of the patients had received taxane and/or anthracycline anticancer drugs as prior therapy; however, they were judged to be non-responsive to the drugs. The size of the targeted tumor decreased to 55.7% after 3 courses of the therapy in Patient 1. The tumor disappeared at completion of the third course in Patient 2, although the therapy was temporarily suspended during the second course due to adverse drug reactions, and the therapy was then resumed after 2-week drug withdrawal. Patient 3 was able to undergo long-term therapy, consisting of 8 courses for 11 months, and the size of the tumor reduced to 58.1%. No serious adverse drug reactions to S-1 occurred in our 3 patients. It is thought that less toxicity enabled Patient 3 to undergo long-term therapy. We consider S-1 to be a useful anticancer drug for treatment of taxane and/or anthracycline resistant recurrent breast cancer.