Hematopoietic progenitor cells (HPCs) traffic to and are retained in the marrow through the trophic effects of the chemokine stromal cell-derived factor-1α (SDF-1α) binding to its receptor, CXC chemokine receptor 4 (CXCR4). AMD3100 reversibly inhibits SDF-1α/CXCR4 binding, and AMD3100 administration mobilizes CD34⁺ cells into the circulation. We therefore tested the hypotheses that the combination of AMD3100 plus granulocyte colony-stimulating factor (G-CSF) (hereafter A + G) would be superior to G-CSF alone (hereafter G) in mobilizing hematopoietic progenitor cells (HPCs) and that A + G-mobilized cells would engraft as well as G-mobilized cells. The primary objective was to determine whether patients mobilized more progenitor cells per unit of blood volume of apheresis after A + G administration versus G alone. Secondary objectives were to determine whether patients mobilized with A + G compared with G alone required fewer apheresis procedures to reach the target level at least 5 × 10⁶ CD34⁺ cells/kg for transplantation and to determine whether patients mobilized with A + G had at least a 90% success rate of autologous transplantation as assessed by neutrophil engraftment by day 21. Each patient served as his or her own control in a sequential mobilization design. All study objectives were met without significant toxicity. The results demonstrate that the combination of A + G is generally safe, effective, and superior to G alone for autologous HPC mobilization. (Blood. 2005;106:1867-1874)