We report a clinical pilot study conducted in 6 women with poor-prognosis breast cancer. The goal was to evaluate the feasibility and safety of producing hematopoietic progenitors and cells from a small marrow sample, for clinical use after high-dose cyclophosphamide. A small volume marrow collection was obtained, using local anesthesia and conscious sedation, before the first of two chemotherapy cycles. Cells were cryopreserved, and later thawed to inoculate two Aastrom Biosciences Inc Replicell bioreactors, on time to reinfuse ex vivo expanded cells after the second chemotherapy cycle. Patients recovered neutrophils and platelets at similar times after the first and second chemotherapy cycles, and showed comparable clinical events. This pilot study prepares future randomized trials, designed to evaluate clinical benefits associated with the use of ex vivo expanded cells in the setting of multicycle high-dose chemotherapy.