G-CSF is given after autologous progenitor cell transplantation to accelerate neutrophil engraftment. Historically, G-CSF has been started on the day of progenitor cell infusion. To study the timing of the initiation of G-CSF after autologous peripheral blood progenitor cell (PBPC) transplantation, we conducted a prospective, randomized trial comparing the initiation of G-CSF therapy on day 0, day +3 or day +5 after autologous PBPC transplantation. Seventy patients with diagnoses of breast cancer, non-Hodgkin’s lymphoma, Hodgkin’s disease, or multiple myeloma were prospectively randomized to one of the three treatment arms. All patients were treated with a chemotherapy (only) preparative regimen. The source of hematopoietic reconstitution was PBPC alone (without autologous marrow), and all patients yielded a minimum of 2 × 106 CD34+ cells per kilogram. Times to neutrophil engraftment and platelet engraftment were identical in the three treatment groups, with neutrophil engraftment occurring at a median of 10, 11 and 11 days when starting G-CSF on day 0, day 3 or day 5, respectively. Time to platelet transfusion independence was 14, 11 and 14 days by treatment group. We conclude that delaying the initiation of G-CSF from day 0 to day +5 does not affect engraftment and results in cost savings.