Summary:Peripheral blood progenitor cells are now commonly used for hematologic reconstitution after myelosuppressive chemotherapy for hematologic and solid malignancies. The purpose of this study was to evaluate the activity of paclitaxel 170 mg/m 2 and cyclophosphamide 2 g/m 2 (CP) with filgrastim (human G-CSF) for mobilization of PBPCs as the first or second maneuver after failure with filgrastim alone. Sixty-four patients with stage II-IV breast cancer received (CP) followed by filgrastim (10 g/kg/day). In 35 (55%) this was the first maneuver while it was for salvage in 29 (45%) patients. The median number of aphereses was two (range, 1-7). In 83% of the patients apheresis was initiated on days 10-11 following chemotherapy. The median numbers of CD34 + cells/kg, CD34 + cells/apheresis/kg and total nucleated cells/kg collected were 8.7 × 10 6 (2.11-73.5), 3.97 × 10 6 (0.3-36.75) and 164.15 × 10 8 (9-660), respectively. All the patients yielded at least 2 × 10 6 CD34 + cells/kg. CP mobilization salvaged the 29 patients who failed mobilization with filgrastim alone. When used as first-line mobilization the yield of CD34 + cells × 10 6 /kg was higher than in the salvage group (16.93 vs 3.94, P Ͻ 0.001). Patients receiving CP as salvage reached the target of 5 × 10 6 CD34 + cells/kg in only 45% (13/29) of cases vs 94.3% as first maneuver. CP followed by filgrastim is a safe and effective regimen for the mobilization of PBPCs in patients with breast cancer and shows significant activity in patients who failed to mobilize with filgrastim, suggesting a higher mobilization potential.