DISEASE CHARACTERISTICS: Metastatic breast cancer Bidimensionally measurable or evaluable disease No bone metastases as only site of measurable or evaluable disease that has been receiving bisphosphonate therapy for less than 2 months No known evidence of brain metastases, lymphangitis lung metastases, or carcinomatous meningitis Hormone receptor status: Not specified

PATIENT CHARACTERISTICS: Age: 18 and over Sex: Female Menopausal status: Not specified Performance status: ECOG 0-1 Life expectancy: At least 3 months Hematopoietic: WBC at least 3,000/mm3 Granulocyte count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.5 mg/dL AST or ALT no greater than 2.5 times upper limit of normal (ULN) Renal: Creatinine no greater than 1.5 times ULN Calcium no greater than 1.3 times ULN Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception No history of other cancers except curatively treated carcinoma in situ of the cervix or nonmelanomatous skin cancer No active serious infection or other serious underlying medical condition that would preclude study therapy No dementia or significantly altered mental status that would preclude study consent No known hypersensitivity to fluorouracil-uracil or leucovorin calcium

PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent immunotherapy Concurrent colony stimulating factors (e.g., filgrastim (G-CSF), sargramostim (GM-CSF)) allowed only during time off treatment during each course Chemotherapy: No prior chemotherapy for metastatic disease At least 6 months since prior adjuvant chemotherapy and recovered Prior adjuvant fluorouracil allowed provided not infusional No prior fluorouracil-uracil with or without leucovorin calcium, capecitabine, S-1, or ethynyl uracil No other concurrent chemotherapy Endocrine therapy: Prior hormonal therapy for metastatic disease or in adjuvant setting allowed Recovered from toxicity No concurrent hormonal anticancer therapy Radiotherapy: Prior radiotherapy for metastatic disease or in adjuvant setting allowed At least 2 weeks since prior radiotherapy and recovered No prior radiotherapy to greater than 30% of bone marrow No concurrent radiotherapy except for palliation of painful bone metastases, pathologic fractures of known lytic disease, or brain lesions that develop Surgery: Not specified Other: No other concurrent investigational therapy No other concurrent anticancer drugs No concurrent halogenated antiviral agents (e.g., lodenosine, fialuridine, L-FMAU, emtricitabine, or sorivudine) No concurrent initiation of bisphosphonate therapy for development of new bone lesions or progression of existing bone lesions