The feasibility and safety of the administration of multiple cycles of dose-intensive chemotherapy (CT) supported with repeated reinfusions of circulating progenitor cells (CPCs) were evaluated in a prospective study of adjuvant initial therapy of poor-prognosis breast cancer. Eighteen patients with resectable breast cancer involving >/= 10 axillary nodes or >/= 5 axillary nodes and negativity of the estrogen receptor status received a cycle of standard FEC regimen (5-FU 600 mg/m2, epirubicin 60 mg/m2, CTX 600 mg/m2, i.v. on day 1) followed by G-CSF as CPC mobilization technique. Collected CPCs were fractionated and reinfused, with G-CSF, after each of the 4 subsequent cycles of high-dose FEC (HD-FEC) (5-FU 750 mg/m2, epirubicin 120 mg/m2, CTX 3 g/m2, i.v.) planned at 21 day intervals. The median numbers of CD34+ cells and CFU-GM collected (with one or two leukaphereses per patient) were 9.7x10(6)/kg (range: 2.5-22.9) and 9.9x10(4)/kg (range: 1.9-27.3), respectively, and day 9 was the median first day of procedure (range: 8-12) after FEC. All patients received the 4 courses of HD-FEC (for a total of 72 cycles). Hemopoietic recovery was rapid after each cycle and there was no treatment-related delays in CT administration. Mucositis was the major non-hematological toxicity. There were 2, 3, 7 and 9 episodes of WHO grade 3/4 mucositis in cycles 1, 2, 3 and 4, respectively. These severe episodes lasted a median of 4 days (range: 2-6) but no patient required parenteral nutrition. The mean +/- SD total hospital stay lasted 10 +/- 2 days. The delivery of 4 cycles of dose-intensive FEC CT supported by CPCs (mobilized with a single course of standard-dose FEC + G-CSF) is feasible and safe. It could represent an effective alternative strategy to other more aggressive programs for the adjuvant therapy of high-risk early breast cancer.