Previous phase II studies of continuous infusion Fluorouracil (5-FU) (CI 5-FU) in refractory metastatic breast cancer have shown modest activity with low toxicity. Its activity in a first-line setting has not been formally tested. Patients were eligible if they fulfilled the following criteria: metastatic breast cancer; measurable or evaluable disease, no prior chemotherapy in the metastatic setting; ECOG performance status of 0, 1, or 2: adequate bone marrow and liver function. Patients were treated with 5-FU 250 mg/m2 per day by continuous intravenous infusion for 5 weeks in a 6-week cycle. Treatment was continued until disease progression or unacceptable toxicity. In addition to the traditional endpoints of response, survival, and toxicity, quality of life was assessed with the Functional Living Index-Cancer (FLIC) and the Symptom Distress Scale (SDS). Twenty-one patients were enrolled. Among the 16 patients with measurable disease, the objective response rate was 44% (95% CI 20%, 68%) with CR rate 13% and PR rate 31%. The median duration of response was 37 weeks. Responses were not observed in patients with visceral (lung or liver) disease. Among all 21 patients in the study, the median time to disease progression was 12 weeks, and median overall survival was 64 weeks. Grade 1 or 2 mucosal and cutaneous toxicity were common. Only 4 patients (19%) had toxicity greater than grade 2; three patients had grade 3 mucositis, and 1 patient developed an indwelling catheter infection requiring its removal. Among responding patients, mean FLIC scores improved from 114.3 at baseline to 128.7 at week 8 (p = 0.11). Symptoms reported on the SDS generally improved in responding patients. Continuous infusion 5-FU as a first-line therapy for metastatic breast cancer has moderate activity and low toxicity. Its use should be considered in the first-line setting when toxicity needs to be minimized.