Myeloid engraftment after bone marrow transplantation (BMT) is influenced by a number of variables, including cytoreductive chemoradiotherapy, genetic disparity, number of reinfused committed myeloid progenitor cells, healthy microenvironment, and the presence of hematopoietic growth factors. Granulocyte colony-stimulating factor (G-CSF) stimulates proliferation of myeloid progenitor cells and enhances myeloid engraftment after BMT. We investigated the temporal relationship between endogenous G-CSF production and myeloid engraftment in both children and adults after allogeneic (ALLO) and autologous (AUTO) BMT. Circulating endogenous G-CSF levels ranged between 0 and 2552 pg/mL. The correlation coefficient between circulating serum G-CSF levels and the peripheral absolute neutrophil count (ANC) was r = -.875 (P less than .001). The endogenous serum G-CSF level was highest during the first week after BMT, when the ANC was less than or equal to 200/microL (699 +/- 82.3 pg/mL) (P less than .001). Both children and adults demonstrated a similar inverse relationship between circulating G-CSF level and degree of neutropenia. One patient failed to engraft after AUTO BMT and also failed to generate any endogenous G-CSF production. Lastly, once the serum G-CSF level decreased to less than 200 pg/mL, a mean of 6.1 +/- 0.9 days elapsed before the ANC was greater than or equal to 500/microL for 2 consecutive days. This study demonstrates that endogenous G-CSF production is associated with myeloid engraftment in both children and adults after AUTO and ALLO BMT and that the rate of increase and decrease in endogenous G-CSF may be predictive of either failure to engraft or duration of neutropenia.