The number and growth factor requirements of committed progenitor cells (colony-forming units-granulocyte/macrophage and burst-forming units- erythroid) in three patients with cyclic neutropenia (two congenital, one acquired) were studied before and during therapy with recombinant human granulocyte colony-stimulating factor (G-CSF; 3 to 10 micrograms/kg/d). When the patients with congenital disease were treated with G-CSF, the cycling of blood cells persisted, but the cycle length was shortened from 21 days to 14 days, and the amplitude of variations in blood counts increased. There was a parallel shortening of the cycle and increase of the amplitude of variations (from two- to three-fold to 10- to 100-fold) in the number of both types of circulating progenitor cells in these two patients. In the patient with acquired cyclic neutropenia, cycling of both blood cells and progenitors could not be seen. In cultures deprived of fetal bovine serum, erythroid and myeloid bone marrow progenitor cells from untreated patients and from normals differed in growth factor responsiveness. As examples, maximal growth of granulocyte/macrophage (GM) colonies was induced by granulocyte/macrophage (GM)-CSF plus G-CSF in the patients, whereas a combination of GM-CSF, G-CSF and interleukin- 3 (IL-3) was required in the normals, and erythropoietin alone induced fourfold more erythroid bursts from cyclic neutropenic patients than from normal donors (46% versus 11% of the maximal colony number, respectively). The growth factor responsiveness of marrow progenitor cells slightly changed during the treatment toward the values observed with normal progenitors. These results indicate that treatment with G- CSF not only ameliorated the neutropenia, but also increased the amplitude and the frequency of oscillation of circulating progenitor cell numbers. These data are consistent with the hypothesis that G-CSF therapy affects the proliferation of the hematopoietic stem cell.