There has been no previously published experience with granulocyte-macrophage colony stimulating factor (GM-CSF) doses less than 12 micrograms/m2 daily in patients with myelodysplastic syndromes, and most observations have been made at doses greater than or equal to 120 micrograms/m2 daily. We administered 5 micrograms/m2 daily by subcutaneous injection to 29 such patients increasing the dose in patients who did not show a haematologic response. Doses of 5 or 10 micrograms/m2 ('low-dose GM-CSF') produced an increase in neutrophils in 14/29 patients. Response was significantly (P = 0.03) more frequent in patients who had a higher pre-treatment neutrophil count (e.g. 11/16 in patients with greater than or equal to 0.5 x 10(9)/l). A rise in blasts followed administration of low-dose GM-CSF in five patients, all with either refractory anaemia with excess blasts (RAEB) or refractory anaemia with excess blasts in transformation (RAEBT). Platelets decreased in five patients, four of whom had no change in blasts, reverting to baseline when GM-CSF was discontinued. We and others have previously observed similar rises in blasts or decreases in platelets at doses of 120 micrograms/m2 daily. Low-dose GM-CSF produced no constitutional side effects. Our results suggest that low doses of GM-CSF might be initially employed in neutropenic patients with myelodysplastic syndromes who present with pretreatment neutrophil counts greater than 0.5 x 10(9)/l. Increasing the dose, and hence the risk of extramedullary toxicity, only in patients who do not respond to the low dose. Patients who present with lower pre-treatment neutrophil counts might begin treatment at doses above 10 micrograms/m2, but below the 120 micrograms/m2 commonly employed, which may be necessary in relatively few patients.