Hematopoietic growth factors like G-CSF or GM-CSF have been shown to shorten the period of severe neutropenia after HD chemotherapy and autologous BMT, and are now widely used to mobilize hemopoietic stem cells into peripheral blood. In order to evaluate the possibility of delaying G-CSF administration after transplantation of G-CSF mobilized blood stem cells (BSC), we randomized 35 cancer patients to receive CSF at day 1 (group 1, n = 19) or at day 6 (group 2, n = 16) after transplantation and here we present their hematological reconstitution. BSC collection was performed by apheresis after G-CSF priming for 5 or 6 days (600 micrograms daily subcutaneously). Hematological recovery is comparable between the two groups: a median of 10 (range 7-16) vs 11 (range 9-18) days to reach an ANC > 0.5 x 10(9)/1 in group 1 (G-CSF day 1 after transplant) vs group 2 (G-CSF day 6 after transplant, P = NS). Median time to reach an unsupported platelet count of 25 x 10(9)/1 was 14 days in the two groups (range 8-110 and 10-40 respectively, P = NS); patients received less G-CSF after transplantation in group 2. No difference appeared in terms of transfusion support, number of days of fever of i.v. antibiotic treatment. Patients' hospital stay was the same in the two groups. Our data suggest that delaying G-CSF administration after infusion of mobilized blood cells is not detrimental to hematological recovery, while it lowers the overall cost of the procedure.