We studied the effect of granulocyte colony-stimulating factor on the magnitude of peripheral blood stem cell mobilization in patients with malignancy. The leukapheresis products mobilized with granulocyte colony-stimulating factor alone at a steady state (a period of full hematopoietic recovery) (group 1) were compared with those obtained after cytotoxic chemotherapy using granulocyte colony-stimulating factor (group 2). In group 1, six patients underwent six courses of stem cell collection with a median of 20 l leukapheresis. In group 2, 10 patients underwent 12 courses of stem cell collection with a median of 10 l leukapheresis. Median yields of group 1 vs. group 2 were mononuclear cells (x 10(9)), 21.9 vs. 11.6; CD34+ cells (x 10(6)/l), 14.5 vs. 17.1; colony-forming unit for granulocyte-macrophage (/ml), 223 vs. 1193; burst-forming unit for erythroid (/ml), 29 vs. 71; colony-forming unit for erythroid (/ml), 42 vs. 29; colony-forming unit for megakaryocyte (/ml), 26 vs. 59. While there were no statistically significant differences in the number of CD34+ cells between the two groups, granulocyte-macrophage-committed progenitor cells were more enriched in the apheresis products of group 2. The correlation between CD34+ cells and colony-forming unit for granulocyte-macrophage was poor. Our results demonstrate that granulocyte colony-stimulating factor can mobilize a sufficient number of progenitor cells into the peripheral blood for stem cell transplantation with or without prior chemotherapy.