A phase II study of 5'-deoxy-5-fluorouridine (5'-DFUR) tablets for breast cancer was done at 31 institutions in Japan. Forty-five patients were registered and 44 of them were eligible for the study. Of the 40 patients whose results could be evaluated, 11 (28%) responded (four complete responses and seven partial responses). Side effects, such as diarrhea, anorexia, leukocytopenia, and liver dysfunction were observed in 24 of the 44 patients. The side effects were mild and transient. 5'-DFUR is a promising drug for breast cancer treatment, and its tablet form makes chemotherapy easier for the patient.