Granulocyte-macrophage colony-stimulating factor (GM-CSF), a pleiotropic molecule which displays a broad range of haematopoietic activities, has become available for clinical evaluation in various patient groups. It has been shown to be effective in preventing or reversing neutropenia. Adverse effects of GM-CSF, however, are dose related. Appropriate dose, route and schedules for GM-CSF in various clinical settings have recently been defined, the usual range being 5-10 micrograms/kg/day either by 4-6 h intravenous infusion or by subcutaneous injection. At such doses, adverse effects are predominantly mild-to-moderate in nature, occur in 20-30% of patients and usually comprise fever, myalgia, malaise, rash and injection site reaction. Early trials using very high doses of GM-CSF were often associated with marked adverse effects, which in rare cases proved severe (pericarditis and thrombosis). Similarly, a so-called "first-dose reaction", defined as a syndrome of hypoxia and hypotension after the initial but not subsequent doses of GM-CSF, was observed in certain predisposed patients following doses above 10 micrograms/kg/day. Subsequent trials have established that intravenous bolus or short infusions of GM-CSF are more likely to promote adverse effects. Certain patient groups, for example those with myelodysplastic syndrome, acute myeloid leukaemia, inflammatory disease, autoimmune thrombocytopenia or malfunctional immunological responsiveness, require careful clinical monitoring in order to avoid potential complications following the administration of GM-CSF. With the current appropriate administration and doses of GM-CSF, the benefit:risk ratio has been greatly improved.