To verify whether the association of granulocyte-macrophage colony-stimulating factor (GM-CSF) and erythropoietin (EPO) would allow both the acceleration and the dose escalation of the cyclophosphamide/epidoxorubicin/5-fluorouracil (CEF) regimen as first-line therapy in advanced breast cancer patients, we conducted a dose-finding study. Cohorts of three consecutive patients received cyclophosphamide (Ctx, dose range 800-1400 mg/m2), epidoxorubicin (Epidx, dose range 70-100 mg/m2), and 5-fluorouracil (5-Fu, 600 mg/m2, fixed dose) given as an intravenous bolus on day 1 every 14 days; GM-CSF at 5 micrograms/kg given as a subcutaneous injection from day 4 to day 11; and EPO at 150 IU/kg given as a subcutaneous injection three times a week. In no single patient was any dose escalation allowed. A total of 14 patients entered the study. At the 4th dose level (Ctx 1400 mg/m2, Epidx 100 mg/m2, 5-Fu 600 mg/m2), two patients had dose-limiting mucositis and one patient developed dose-limiting neutropenia. Therefore, the 3rd cohort received the maximum tolerated dose, i.e. Ctx at 1200 mg/m2, Epidx at 90 mg/m2, and 5-Fu at 600 mg/m2, given every 18.5 (+/-2.5) days. Toxicity was moderate and manageable in an outpatient setting. Only 1 admission at the 4th dose level was required. Throughout the 4 dose levels there was no toxicity-related death; grade IV leukopenia ranged from 24% to 75% of cycles and grade IV thrombocytopenia ranged from 6% to 8%. No grade IV anemia was recorded. Increasing the doses of Ctx and Epidx while maintaining a fixed dose of 5-Fu with the support of both EPO and GM-CSF allows safe acceleration and dose escalation of CEF chemotherapy. Further controlled studies will evaluate the activity and efficacy of this strategy.