The efficacy of GM-CSF was investigated in 20 neutropenic patients (< 1500 neutrophils/microliters) with acute visceral leishmaniasis due to Leishmania chagasi. Patients were randomized to receive either GM-CSF, 5 micrograms/kg daily (intravenously or subcutaneously), or placebo for ten days, in combination with pentavalent antimony, 10-20 mg/kg daily for 20 days. Neutrophil counts were significantly greater on days 5 and 10 of treatment in the GM-CSF group compared with the placebo group (p < 0.02). Eosinophil and monocyte counts were also significantly increased in the GM-CSF group at day 10 (p < or = 0.03). Interestingly, at day 30, platelet counts were significantly increased in the GM-CSF group on days 5 and 10 (p = 0.04 and 0.02, respectively). Patients in the GM-CSF group experienced fewer secondary bacterial or viral infections than placebo patients. Infections occurred in only three patients given GM-CSF compared with eight patients given placebo (p < 0.04). All patients had complete resolution of disease symptoms at three months. Few adverse events were recorded. GM-CSF given subcutaneously at a dose of 5 micrograms/kg daily for ten days was well tolerated, reversed neutropenia rapidly and reduced the number of secondary infections in patients with leishmaniasis.