Colony stimulating factors (CSF) are now widely used in marrow transplantation. Randomized trials have shown that granulocyte macrophage-CSF accelerates marrow recovery after autologous marrow transplantation, resulting in fewer infections, shorter hospitalization, and lower costs. Similar results have been seen with granulocyte-CSF. Both factors also can accelerate engraftment after allogenic marrow transplantation, but there is, so far, less evidence for clinical benefit in this setting. Both granulocyte macrophage-CSF and granulocyte-CSF benefit some patients with graft failure. The recent demonstration that both granulocyte-CSF and granulocyte macrophage-CSF can mobilize large numbers of hematopoietic stem cells into peripheral blood has resulted in the widespread use of this technique as an alternative to autologous marrow transplantation. A number of colony stimulating factors, including IL-1, IL-3, and steel factor, which act on early hematopoietic progenitors, have recently been entered into clinical trials, with the hope that the progress already made with G-CSF and GM-CSF can be continued.