GM-CSF has been used in clinical trials to assess its role in promoting the proliferation and differentiation of marrow cells and enhancing the functional activities of granulocytes and monocytes. These studies have indicated that GM-CSF may prove useful in the management of cancer patients by preventing or treating myelosuppression following cancer chemotherapy and in patients with myelodysplasia or aplastic anaemia. As well as determining the efficacy of GM-CSF as a therapeutic agent, these studies are also providing insights into the possible roles of GM-CSF in vivo. Pharmacokinetic studies of GM-CSF in patients with advanced cancer and myelodysplasia suggest that the ratio of efficacy to toxicity of GM-CSF can be modified by changing either the dose or the method of administration.