The authors report a phase II pilot investigation in the Southwest Oncology Group examining a combination of 5-fluorouracil, doxorubicin, and cyclophosphamide (FAC) incorporating modulated 5-FU in patients with poor-prognosis stage IV breast cancer. Patients with poor-prognosis stage IV breast cancer were treated with this "neo-FAC" as front-line therapy. The regimen consisted of 5-fluorouracil by continuous ambulatory infusion pump at 200 mg/m2/day for 42 days, repeated at 56-day intervals; doxorubicin at 20 mg/m2/week intravenously to a maximum cumulative total dose (including adjuvant therapy, if any) of 500 mg/m2; cyclophosphamide 60 mg/m2/day taken orally; methotrexate 15 mg/m2/week intravenously beginning 1 week after termination of doxorubicin; and oral prednisone decreasing from 60 mg/day on a tapering schedule for a total of 7 weeks of treatment. Treatment was continued until progression, unacceptable toxicity, or patient refusal. Twenty-four patients were accrued to this study. Of these, two were ineligible, and the remaining 22 were evaluable for response. Ten patients experienced grade 3 toxicity, and six had grade 4. There were no treatment-associated deaths. Best responses were a complete response in one patient (5%) and partial responses in 6, for an overall response rate of 32% (7/22 evaluable patients). Overall survival in five pilot studies in the Southwest Oncology Group in this poor-prognosis population are relatively superimposable. The present regimen, with its relatively poor outcome and the expense and inconvenience of administering chemotherapy by ambulatory infusion pump, will not be pursued further.