The positive role of G-CSF in hastening the myeloid recovery of patients undergoing allogeneic bone marrow transplantation (ALLO-BMT) or autologous bone marrow transplantation (ABMT) has recently been established. Considerable knowledge about adequate doses and route of administration has been accumulated in the past few years. Nonetheless, the optimal time to start growth-factor administration remains undetermined. We have performed a stratified study according to the source of hematopoietic progenitors (ALLO-BMT or ABMT), underlying disease and its stage, and acute graft-versus-host disease (GVHD) prophylaxis regimen and randomized patients in two arms: group A, which started G-CSF on day 0 (36 patients), and group B, which started on day +7 post-BMT (39 patients). The same dose (5 micrograms/kg/day) and route of administration were employed in both groups. We found no significant differences in the time to reach an absolute neutrophil count (ANC) of 0.1, 0.5, and 1 x 10(9)/l and 50 x 10(9) platelets/l (medians: 10 and 11, 14.5 and 14, 17 and 16, 23 and 24 days, respectively, in groups A and B). We did not find differences in the days of fever or days on antibiotic treatment with less than 1 x 10(9)/l ANC, rate of bacteriemia, or days of hospitalization in both groups. In contrast, a considerable saving of G-CSF in B group was found (mean days of infusion in group A, 18, versus 11 in group B) (p < 0.0001). This is equivalent to a saving of 1120 $US per patient.(ABSTRACT TRUNCATED AT 250 WORDS)